Diagnostics_area does not always contain apply error info

It appears that some error conditions don't store error information in the Diagnostics_area. For example when table_def::compatible_with() check fails error message is stored in Relay_log_info instead. This results in optimistically identical votes and zero error buffer size breaks wsrep-lib logic as it relies on error buffer size to decide whether voting took place. To account for this, first try to obtain error info from Relay_log_info, then fallback to Diagnostics_area. If that fails use some random data to distinguish this condition from success. This requires bumping of the application protocol to 8 since vote message generation algorithm has changed.