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/cgo.go

@@ -0,0 +1,15 @@
+package main
+
+// #error hello
+// )))
+import "C"
+
+func main() {
+}
+
+// TODO: this error should be relative to the current directory (so cgo.go
+// instead of testdata/errors/cgo.go).
+
+// ERROR: # command-line-arguments
+// ERROR: testdata/errors/cgo.go:3:5: error: hello
+// ERROR: testdata/errors/cgo.go:4:4: error: expected identifier or '('

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'

testdata/errors/types.go

@@ -0,0 +1,12 @@
+package main
+
+func main() {
+	var a int
+	a = "foobar"
+	nonexisting()
+}
+
+// ERROR: # command-line-arguments
+// ERROR: types.go:5:6: cannot use "foobar" (untyped string constant) as int value in assignment
+// ERROR: types.go:6:2: undefined: nonexisting
+// ERROR: types.go:4:6: a declared and not used