New verify-error GnuTLS interface for certificate validation

New verify-error GnuTLS interface for certificate validation

* net/gnutls.el (gnutls-verify-error): New defcustom to control
the behavior when a certificate fails validation.  Defaults to
old behavior: never abort, just warn.
(gnutls-negotiate): Use it.

* gnutls.c: Replace `:verify_hostname_error' with `:verify_error',
now a list of certificate validation checks that will abort a
connection with an error.
(Fgnutls_boot): Document it and use it.