Beside help, error handling is an important aspect of user support. Error handling helps users to overcome problem situations and to continue their work.
Typically, error handling is done by indicating the location where the error occurred and by sending an error message that notes the error, explains the reason for it and - ideally - provides hints on how to remedy the error situation.
For details on message texts see chapter Formulating Messages in the SAP Reference Lists on the SAP Design Guild.
This page covers three areas:
Error Prevention
Error Handling for Fields
Error Handling in Tables