Item12952: Fix dependency on Dependency

The Department of Redundancy Department renamed Dependency::check() to
Dependency::checkDependency()

Unit tests run, 7 failures remain due to the Configure changes.
Item12952: Really return 500 on internal errors.

Foswiki was returning 200.

There is still an issue here I think.  The Engine returns 500 when the
caller is expecting a request object, and checks for 'defined'.  It
should probably return undef.