all: add testing for compiler error messages

This is needed for some improvements I'm going to make next.

This commit also refactors error handling slightly to make it more
easily testable, this should hopefully not result in any actual changes
in behavior.

testdata/errors/syntax.go

@@ -0,0 +1,7 @@
+package main
+
+func main(var) { // syntax error
+}
+
+// ERROR: # command-line-arguments
+// ERROR: syntax.go:3:11: expected ')', found 'var'