Error: j_id0:j_id1:j_id28:j_id30: Validation Error: Value is required

If you use the label attributed on your selectRadio, that value would be displayed instead of the j-id0.....

    <apex:page standardController="Account" extensions="EventController"> 
<apex:form >
<apex:pageMessages></apex:pageMessages>
<apex:pageBlock >
<p>
Please select 
</p>
<apex:selectRadio value="{!selectType}" label="Choose One" required="true">
 <apex:selectOptions value="{!Option}"/>
</apex:selectRadio>
<apex:commandButton value="Next" action="{!next}"/>
</apex:pageBlock>
</apex:form>
</apex:page>

Each one of the j_id# in the chain represent an Id for an apex tag, if you assign them Ids, then it will show YourIdName:NextIdName:AndSoON

If I use your example and add Ids as so:

<apex:form id="form">
  <apex:pageMessages></apex:pageMessages>
  <apex:pageBlock id="pageBlock">
    <p>Please select </p>
    <apex:selectRadio value="{!selectType}" required="true" id="radioSelectType">
      <apex:selectOptions value="{!Option}"/>
    </apex:selectRadio>
    <apex:commandButton value="Next" action="{!next}" id="buttonNext" />
  </apex:pageBlock>
</apex:form>

The error might be form:pageBlock:radioSelectType and then the error message. As for changing what the error message says, this is only possible (as far as I know) with a controller.