scripts/test_printers_common.py: Log GDB error message

If GDB prints an error message for a "python" command, include
that error message in the test log output, to simplify diagnosing
GDB/Python detection issues.

scripts/test_printers_common.py

@@ -87,6 +87,7 @@ try:
 
     if gdb_python_error:
         print('gdb must have python support to test the pretty printers.')
+        print('gdb output: {!r}'.format(gdb_python_error))
         exit(UNSUPPORTED)
 
     # If everything's ok, spawn the gdb process we'll use for testing.