Payment processor check configuration - improved message in UI

This implements a simple method to check payment processor configuration by executing the existing checkConfig() method and displaying any errors via the Payment processor configuration UI.  Previously checkConfig was only executed (silently) when actually loading the processors on the page and the errors were never used (or seen).  This resulted in payment processors seemingly missing from contribution pages with no indication as to why.

If errors are detected you will now be bounced back to the processor config page (press cancel if you don't wish to configure everything at this point).