NEXUS-6307: JCE warning simplified

Removed complicated static stuff and handling all the
bits in private to crypto plugin.

Also, in case of lack of AES algorithm, letting the
exception bubble up.

Note: this will not prevent Nexus to boot up (as AssertionError
did), but will make Nexus logs look ugly enough to report.
Merge pull request #387 from sonatype/NEXUS-6307-jce-warn-simplified

NEXUS-6307: JCE warning simplified