Error messages are not working well

[atsju]

Code like this won't print the component it tried to search :

    string(REGEX MATCH "^STM32([A-Z][0-9])([0-9A-Z][0-9][A-Z][0-9A-Z])?_?(M[47])?.*$" COMP ${COMP})
    
    if((NOT CMAKE_MATCH_1) AND (NOT CMAKE_MATCH_2))
        message(FATAL_ERROR "Unknown CMSIS component: ${COMP}")
    endif()

This is because COMP is replaced itself by what is found though the regex. But when the regex doesn't match the error should be more explicit. There are plenty of errors that could be added to help new users. This must be improved.