feat(cli-validate): Use argument -v to print errors on stderr

The argument -v (verbosity) is not yet used in the validate function.
The specification of -v is now used in xmlschema-validate to obtain a
detailed error report on the stderr. This helps users to find the error
in their XML files at the mentioned place.

Signed-off-by: Sven Sager <s.sager@kunbus.com>

xmlschema/cli.py

@@ -269,5 +269,8 @@ def validate():
             else:
                 tot_errors += len(errors)
                 sys.stderr.write(f"{filepath} is not valid\n")
+                if args.verbosity > 0:
+                    for error in errors:
+                        sys.stderr.write(f"{error}\n")
 
     sys.exit(tot_errors)