For developers in a CRM on-prem environment, error details can be enabled when encountering CRM SDK organization/discovery service call errors with the following message.

The server was unable to process the request due to an internal error.  For more information about the error, either turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the <serviceDebug> configuration behavior) on the server in order to send the exception information back to the client, or turn on tracing as per the Microsoft .NET Framework SDK documentation and inspect the server trace logs.

On the CRM server, edit: C:\Program Files\Microsoft Dynamics CRM\CRMWeb\web.config

Update the configuration element: <add key="DevErrors" value="On" />

Enable WCF exception details by adding the service behavior configuration:

 <system.serviceModel>
    <serviceHostingEnvironment aspNetCompatibilityEnabled="true" />
    <behaviors>
      <serviceBehaviors>
        <behavior>
          <serviceDebug httpHelpPageEnabled="true" includeExceptionDetailInFaults="true" />
        </behavior>
      </serviceBehaviors>
    </behaviors>
  </system.serviceModel>

The "DevErrors" switch can also be updated (along with other CRM tracing options) with the Diagnostics Tool for Microsoft Dynamics CRM 2011.