cmd/compile/internal/syntax: fix error handling for Read/Parse calls

- define syntax.Error for cleaner error reporting
- abort parsing after first error if no error handler is installed
- make sure to always report the first error, if any
- document behavior of API calls
- while at it: rename ReadXXX -> ParseXXX (clearer)
- adjust cmd/compile noder.go accordingly

Fixes #17774.

Change-Id: I7893eedea454a64acd753e32f7a8bf811ddbb03c
Reviewed-on: https://go-review.googlesource.com/32950
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/dumper_test.go

@@ -14,7 +14,7 @@ func TestDump(t *testing.T) {
 		t.Skip("skipping test in short mode")
 	}
 
-	ast, err := ReadFile(*src, nil, nil, 0)
+	ast, err := ParseFile(*src, nil, nil, 0)
 	if err != nil {
 		t.Fatal(err)
 	}