Allow a context to be passed in for error handling

Allow a context to be passed in for error handling

As pointed out by Tom Lane, we can allow other users of the error
handler callbacks to provide their own memory context by adding
the context to use to ErrorData and using that instead of explicitly
using ErrorContext.

This then allows GetErrorContextStack() to be called from inside
exception handlers, so modify plpgsql to take advantage of that and
add an associated regression test for it.