Don't submit in race-condition check if validations fail

[pixeldesu]

We simply check for HTML validations to pass before finally trying to submit in the submit handler in powermail_cond.

This just triggers reportValidity() on the Powermail form, so if no validations are present in HTML5, everything should work fine as well.

Testing:

• Use a form like https://powermailcond-v13.ddev.site/powermail/powermail-cond/show-fieldset-invoice-address and make a field mandatory (e-mail is a good one), try to submit the form without a mail address set.
• It should show a validation error and not actually submit.

Fixes #111