Improve syntax exception reporting

[Stephen Siegel]

There is a nice SyntaxException class, but it is not used in many many places in the front end. Instead, various runtime exceptions are thrown and the error reports are often cryptic.

Someone needs to go through the whole front end and check that all syntax exceptions are dealt with in a common way : throwing a SyntaxException with a non-null Source info and good, clear message. The exceptions are caught up in the verify module and reported correctly.

[zirkel]

FYI: I made front-end expressions sourceable, so now they can be passed to a SyntaxException.

[bperry]

Runtime exceptions and System.exit()'s have been changed to SyntaxExceptions.