Exception: increase verbosity with imap_alerts() and imap_errors()

[Slamdunk]

Call imap_alerts() and imap_errors() while building Exception message to increase verbosity and raise awareness of what happened.

Before:

Authentication failed for user "fake_username_59c9f91cc66e6": imap_open():
Couldn't open stream {my.server.com:60993/imap/ssl/validate-cert}

After:

[E_WARNING] Authentication failed for user "fake_username_59c9f91cc66e6": imap_open():
Couldn't open stream {my.server.com:60993/imap/ssl/validate-cert}
imap_alerts (0):
imap_errors (1):
- Can not authenticate to IMAP server: [AUTHENTICATIONFAILED] Authentication failed.

[Slamdunk]

Don't repeat yourself: with final keyword we ensure every Exception in this library follows this pattern

[Slamdunk]

As you can see in Travis in the first build of this PR:
https://travis-ci.org/ddeboer/imap/jobs/279831280#L722-L726

1) Ddeboer\Imap\Tests\ServerTest::testFailedAuthenticate
Failed asserting that exception message '[E_WARNING] Authentication failed for user "fake_username_59c9fb455ebfb": imap_open(): Couldn't open stream {dovecot.travis.dev:993/imap/ssl/validate-cert}
imap_alerts (0):
imap_errors (1):
- Certificate failure for dovecot.travis.dev: self signed certificate: /C=EU/ST=Europe/L=Home/O=Travis/OU=Travis DEV/CN=dovecot.travis.dev' matches '/E_WARNING.+AUTHENTICATIONFAILED/s'.

It is far easier to detect the real error now.