Fix error check in gdb_py_test_silent_cmd

I added a new test using gdb_py_test_silent_cmd, and then was
surprised to find out that the new test passed -- it caused a Python
exception and I had expected it to fail.  This patch fixes this proc
to detect this situation and fail.

gdb/testsuite/lib/gdb-python.exp

@@ -22,6 +22,7 @@ proc gdb_py_test_silent_cmd { cmd name report_pass } {
     global gdb_prompt
 
     gdb_test_multiple $cmd $name {
+	-re "Error occurred in Python:.*$gdb_prompt $" { fail $name }
 	-re "Traceback.*$gdb_prompt $" { fail $name }
 	-re "$gdb_prompt $"            { if $report_pass { pass $name } }
     }