ERR/E5.1: lambda-specific raise-not-failable hint

A closure literal whose body raises but is annotated non-failable (or has
no ! in its return) now gets a lambda-specific diagnostic telling the user
to declare the failable return explicitly, instead of the generic "raise
is only valid inside a failable function". Failability is never inferred
for a lambda, so a raising lambda with no ! is a hard error that should
point at the fix.

New in_lambda_body flag (save/restore for nesting) set around the lambda
body lowering in lowerLambda; diagRaiseNotFailable branches on it.
Top-level functions keep the generic message.

Test: 1043-errors-lambda-raise-annotation-hint.sx.

examples/expected/1043-errors-lambda-raise-annotation-hint.exit

@@ -0,0 +1 @@
+1