Print a better error message in regression tests on an invalid handle.
Print a better error message in regression tests on an invalid handle.

Has no effect at the moment, but I've seen it happen during development
of new regression tests, when I've passed an invalid handle to print_diag
by accident.

treasure-data/prestogres-odbc,hlinnaka/psqlodbc,gavioto/psqlodbc,gavioto/psqlodbc,hlinnaka/psqlodbc,treasure-data/prestogres-odbc,gavioto/psqlodbc,hlinnaka/psqlodbc,treasure-data/prestogres-odbc