Fix php-yubico Curl ssl support error - PEAR error is deprecated

Using exit() is not effective as Roundcube defaults to an authenticated
state. Also, php-yubico is using PEAR::error which is deprecated and
such error, when in the constructor, will not be catched. This revision
revert to throwing standard exceptions. Hopefully, php-yubico will follow
the same path.
Merge branch 'master' of github.com:northox/roundcube-yubikey-plugin