Improve tooltips and error messages

* Make the error message for an invalid name more explicit now that the
default rules are simpler.
* Make the tool tip for the email address less verbose. It should be
clear that a valid email address needs to be entered.
* If an additional regular expression has been configured for name or
email address, then, in case of an invalid entry, mention that those
organizational rules might not be satisfied.

GnuPG-bug-id: 5916