Add option to silence output message for "-E compare_files" if files differ

Add option to silence output message for "-E compare_files" if files differ. Current there is always a message

Files "xxx" to "yyy" are different.

This message seem not really helpful in scripts, because that is no really an error in man cases. It's something that is expected and it should be up to the script to print some message in this case. Furthermore, when e.g. using

execute_process(
    COMMAND ${CMAKE_COMMAND} -E compare_files ${FILE_1} ${FILE_2}
    RESULT_VARIABLE compare_result
    ERROR_QUIET

)

specifying ERROR_QUIET seem wrong, because I want to see all errors actually - just not the expected error that the file are different