After some testing with endusers it turned out that there are browsers
where OpenPGPjs doesn't work. (This is documented, I just decided to
ignore that.) This resulted in a plaintext mail sent to me, which is
definitely against the purpose of this page. Therefore I added a
failsafe mode that prevents such user error by hiding the form fields
until the JS could verify that the crypto operations work. Additionally
when any error appears, the entire from is hidden and replaced by an
error message, which should prevent such a behavior in future.