cmd/compile: better error for malformed packages

Fixes #14270.

Change-Id: Iaf2496a24b1aba443391b90951cef472add73f13
Reviewed-on: https://go-review.googlesource.com/45950
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -964,7 +964,8 @@ func importfile(f *Val) *types.Pkg {
 	// check object header
 	p, err := imp.ReadString('\n')
 	if err != nil {
-		log.Fatalf("reading input: %v", err)
+		yyerror("import %s: reading input: %v", file, err)
+		errorexit()
 	}
 	if len(p) > 0 {
 		p = p[:len(p)-1]
@@ -979,7 +980,8 @@ func importfile(f *Val) *types.Pkg {
 		}
 		p, err = imp.ReadString('\n')
 		if err != nil {
-			log.Fatalf("reading input: %v", err)
+			yyerror("import %s: reading input: %v", file, err)
+			errorexit()
 		}
 		if len(p) > 0 {
 			p = p[:len(p)-1]
@@ -1004,7 +1006,8 @@ func importfile(f *Val) *types.Pkg {
 	for {
 		p, err = imp.ReadString('\n')
 		if err != nil {
-			log.Fatalf("reading input: %v", err)
+			yyerror("import %s: reading input: %v", file, err)
+			errorexit()
 		}
 		if p == "\n" {
 			break // header ends with blank line