Improve error message

Improve error message for when input cannot be parsed. This is
in response to strange error messages resulting from binary proto being
sent to pb when json was expected.

cmd/pb/main.go

@@ -67,7 +67,7 @@ func run(cfg PBConfig) error {
 	}
 	unmarshal, err := cfg.unmarshaler()
 	if err != nil {
-		return err
+		return fmt.Errorf("cannot decode %q input: %w", cfg.inFormat(), err)
 	}
 	message := dynamicpb.NewMessage(md)
 	if err := unmarshal(in, message); err != nil {
@@ -106,8 +106,7 @@ func (c *PBConfig) writeOutput(b []byte) error {
 }
 
 func (c *PBConfig) unmarshaler() (unmarshaler, error) {
-	format := getFormat(c.In, c.InFormat)
-	switch format {
+	switch c.inFormat() {
 	case "json":
 		o := protojson.UnmarshalOptions{Resolver: c.Protoset}
 		return o.Unmarshal, nil
@@ -118,7 +117,11 @@ func (c *PBConfig) unmarshaler() (unmarshaler, error) {
 		o := prototext.UnmarshalOptions{Resolver: c.Protoset}
 		return o.Unmarshal, nil
 	}
-	return nil, fmt.Errorf("unknown input format %s", format)
+	return nil, fmt.Errorf("unknown input format %q", c.inFormat())
+}
+
+func (c *PBConfig) inFormat() string {
+	return getFormat(c.In, c.InFormat)
 }
 
 func (c *PBConfig) marshaler() (marshaler, error) {