partial fix for bug 31724 emits file:line numbers for declare warning/error.

partial fix for bug 31724 emits file:line numbers for declare warning/error.

This adds an optional "context" String to IMessage.
The IMessage creator should create a context String (or use an implementation that lazily creates one).
The IMessage client can render their messages without context embedded in the message field.

Emitting source context from the weaver will be harder.
Using file and start/end line will probably work, since tool clients can map to any available source file paths.
(BcelSourceContext and BcelShadow can be updated with end from range.)