Give a human readable Warning when SSL Pinning fails.

Encryption, SSL and Certificates are hard to gasp.
And generally errors like "The operation couldn’t be completed.
(NSURLErrorDomain error -1012.)" are hard to make sense of.

I subscribe to the theory that if you enable SSL Pinning
and it fails, something went horribly wrong
a) you misconfigured the thing, a certificate expired or
   some other developer screw up.
b) your application is unter attack.

Both secenarios warrant a clear and obvious message in the
log files. This breaks with the "no logging" approach in
AFURLConnectionOperation and maybe an `NSAssert` would
be better but if SSP pinning fails something fishy is going
on and we should leave some obvious evidence about this.
Merge pull request #1451 from mdornseif/pinningWarning_1.x

1.x: Give a human readable Warning when SSL Pinning fails.