cmd/compile: don't panic if syntax.ReadFile returns an error

Fixes #17772

Change-Id: I0f2094400c454828aa57a8d172dadeac4ddb6d35
Reviewed-on: https://go-review.googlesource.com/32691Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/noder.go

@@ -17,7 +17,8 @@ func parseFile(filename string) {
 	p := noder{baseline: lexlineno}
 	file, err := syntax.ReadFile(filename, p.error, p.pragma, 0)
 	if err != nil {
-		Fatalf("syntax.ReadFile %s: %v", filename, err)
+		fmt.Printf("parse %s: %v\n", filename, err)
+		errorexit()
 	}
 
 	p.file(file)