gps: export some errors for checking/supression from client

Signed-off-by: deadprogram <ron@hybridgroup.com>

gps/gps.go

@@ -11,16 +11,15 @@ import (
 )
 
 var (
-	errInvalidNMEASentenceLength = errors.New("invalid NMEA sentence length")
-	errInvalidNMEAChecksum       = errors.New("invalid NMEA sentence checksum")
-	errEmptyNMEASentence         = errors.New("cannot parse empty NMEA sentence")
-	errUnknownNMEASentence       = errors.New("unsupported NMEA sentence type")
+	ErrInvalidNMEASentenceLength = errors.New("invalid NMEA sentence length")
+	ErrInvalidNMEASentence       = errors.New("invalid NMEA sentence format")
+	ErrEmptyNMEASentence         = errors.New("cannot parse empty NMEA sentence")
+	ErrUnknownNMEASentence       = errors.New("unsupported NMEA sentence type")
 	errInvalidGGASentence        = errors.New("invalid GGA NMEA sentence")
 	errInvalidRMCSentence        = errors.New("invalid RMC NMEA sentence")
 	errInvalidGLLSentence        = errors.New("invalid GLL NMEA sentence")
 	errGPSCommandRejected        = errors.New("GPS command rejected (NAK)")
 	errNoACKToGPSCommand         = errors.New("no ACK to GPS command")
-	errInvalidNMEASentanceFormat = errors.New("invalid NMEA sentence format")
 )
 
 const (
@@ -151,15 +150,15 @@ func (gps *Device) WriteBytes(bytes []byte) {
 // It has to end with a '*' character following by a checksum.
 func validSentence(sentence string) error {
 	if len(sentence) < minimumNMEALength || sentence[0] != startingDelimiter || sentence[len(sentence)-3] != checksumDelimiter {
-		return errInvalidNMEASentenceLength
+		return ErrInvalidNMEASentenceLength
 	}
 	var cs byte = 0
 	for i := 1; i < len(sentence)-3; i++ {
 		cs ^= sentence[i]
 	}
 	checksum := strings.ToUpper(hex.EncodeToString([]byte{cs}))
-	if checksum != sentence[len(sentence)-2:len(sentence)] {
-		return errInvalidNMEASentanceFormat
+	if checksum != sentence[len(sentence)-2:] {
+		return ErrInvalidNMEASentence
 	}
 
 	return nil