Support multiline error/warning reports

Currently reports have no proper output if they have source-references spanning multiple lines.

Maybe even some improvements to the current single line reports could be done.

void main () {
	try {
	} catch (Error e) {
	}
}

$ valac -C test.vala
test.vala:3.10-4.2: warning: unreachable catch clause detected
Compilation succeeded - 1 warning(s)