Context given to EditEntity must be RequestContext or DerivativeContext

check that $context is valid.  EditEntity uses $context->setTitle()
which is not part of IContextSource.

If wrong type of context is given, things fail badly.

Change-Id: I83d12a969b10f0815b7766802d98506fdd12885f
Merge "Context given to EditEntity must be RequestContext or DerivativeContext"