how can I show customized error messaged from server side validation in React Admin package?

If you're using SimpleForm, you can use asyncValidate together with asyncBlurFields as suggested in a comment in issue 97. I didn't use SimpleForm, so this is all I can tell you about that.

I've used a simple form. And you can use server-side validation there as well. Here's how I've done it. A complete and working example.

import React from 'react';
import PropTypes from 'prop-types';
import { Field, propTypes, reduxForm, SubmissionError } from 'redux-form';
import { connect } from 'react-redux';
import compose from 'recompose/compose';
import { CardActions } from 'material-ui/Card';
import Button from 'material-ui/Button';
import TextField from 'material-ui/TextField';
import { CircularProgress } from 'material-ui/Progress';
import { CREATE, translate } from 'ra-core';
import { dataProvider } from '../../providers'; // <-- Make sure to import yours!

const renderInput = ({
    meta: { touched, error } = {},
    input: { ...inputProps },
    ...props
}) => (
    <TextField
        error={!!(touched && error)}
        helperText={touched && error}
        {...inputProps}
        {...props}
        fullWidth
    />
);

/**
 * Inspired by
 * - https://redux-form.com/6.4.3/examples/submitvalidation/
 * - https://marmelab.com/react-admin/Actions.html#using-a-data-provider-instead-of-fetch
 */
const submit = data =>
    dataProvider(CREATE, 'things', { data: { ...data } }).catch(e => {
        const payLoadKeys = Object.keys(data);
        const errorKey = payLoadKeys.length === 1 ? payLoadKeys[0] : '_error';
        // Here I set the error either on the key by the name of the field
        // if there was just 1 field in the payload.
        // The `Field` with the same `name` in the `form` wil have
        // the `helperText` shown.
        // When multiple fields where present in the payload, the error  message is set on the _error key, making the general error visible.
        const errorObject = {
            [errorKey]: e.message,
        };
        throw new SubmissionError(errorObject);
    });

const MyForm = ({ isLoading, handleSubmit, error, translate }) => (
    <form onSubmit={handleSubmit(submit)}>
        <div>
            <div>
                <Field
                    name="email"
                    component={renderInput}
                    label="Email"
                    disabled={isLoading}
                />
            </div>
        </div>
        <CardActions>
            <Button
                variant="raised"
                type="submit"
                color="primary"
                disabled={isLoading}
            >
                {isLoading && <CircularProgress size={25} thickness={2} />}
                Signin
            </Button>
            {error && <strong>General error: {translate(error)}</strong>}
        </CardActions>
    </form>
);
MyForm.propTypes = {
    ...propTypes,
    classes: PropTypes.object,
    redirectTo: PropTypes.string,
};

const mapStateToProps = state => ({ isLoading: state.admin.loading > 0 });

const enhance = compose(
    translate,
    connect(mapStateToProps),
    reduxForm({
        form: 'aFormName',
        validate: (values, props) => {
            const errors = {};
            const { translate } = props;
            if (!values.email)
                errors.email = translate('ra.validation.required');
            return errors;
        },
    })
);

export default enhance(MyForm);

If the code needs further explanation, drop a comment below and I'll try to elaborate.

I hoped to be able to do the action of the REST-request by dispatching an action with onSuccess and onFailure side effects as described here, but I couldn't get that to work together with SubmissionError.

Here one more solution from official repo. https://github.com/marmelab/react-admin/pull/871 You need to import HttpError(message, status, body) in DataProvider and throw it. Then in errorSaga parse body to redux-form structure. That's it. Enjoy.

Found a working solution for react-admin 3.8.1 that seems to work well.

Here is the reference code

https://codesandbox.io/s/wy7z7q5zx5?file=/index.js:966-979

Example:

First make the helper functions as necessary.

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
const simpleMemoize = fn => {
  let lastArg;
  let lastResult;
  return arg => {
    if (arg !== lastArg) {
      lastArg = arg;
      lastResult = fn(arg);
    }
    return lastResult;
  };
};

Then the actual validation code

const usernameAvailable = simpleMemoize(async value => {
  if (!value) {
    return "Required";
  }
  await sleep(400);
  if (
    ~["john", "paul", "george", "ringo"].indexOf(value && value.toLowerCase())
  ) {
    return "Username taken!";
  }
});

Finally wire it up to your field:

const validateUserName = [required(), maxLength(10), abbrevUnique];

const UserNameInput = (props) => {
    return (
        <TextInput
            label="User Name"
            source="username"
            variant='outlined'
            validate={validateAbbrev}
        >
        </TextInput>);
}