cmd/compile/internal/gc: handle errors from *bio.Writer

Change-Id: I425b5d16149a7a40d888a1ab723b3642eb08224f

Author: mateusz834

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

@@ -47,5 +47,7 @@ func dumpasmhdr() {
 		}
 	}
 
-	b.Close()
+	if err := b.Close(); err != nil {
+		base.Fatalf("%v", err)
+	}
 }

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

@@ -57,7 +57,7 @@ func dumpobj1(outfile string, mode int) {
 		fmt.Printf("can't create %s: %v\n", outfile, err)
 		base.ErrorExit()
 	}
-	defer bout.Close()
+
 	bout.WriteString("!<arch>\n")
 
 	if mode&modeCompilerObj != 0 {
@@ -70,6 +70,12 @@ func dumpobj1(outfile string, mode int) {
 		dumpLinkerObj(bout)
 		finishArchiveEntry(bout, start, "_go_.o")
 	}
+
+	if err := bout.Close(); err != nil {
+		base.FlushErrors()
+		fmt.Printf("error while writing to file %s: %v\n", outfile, err)
+		base.ErrorExit()
+	}
 }
 
 func printObjHeader(bout *bio.Writer) {