gps: export some errors for checking/supression from client

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

examples/gps/uart/main.go

@@ -8,7 +8,6 @@ import (
 )
 
 func main() {
-	println("GPS UART Example")
 	machine.UART1.Configure(machine.UARTConfig{BaudRate: 9600})
 	ublox := gps.NewUART(machine.UART1)
 	parser := gps.NewParser()
@@ -16,14 +15,24 @@ func main() {
 	for {
 		s, err := ublox.NextSentence()
 		if err != nil {
-			println(err)
-			continue
+			switch err {
+			case gps.ErrUnknownNMEASentence, gps.ErrInvalidNMEASentence, gps.ErrInvalidNMEASentenceLength:
+				continue
+			default:
+				println("sentence error:", err)
+				continue
+			}
 		}
 
 		fix, err = parser.Parse(s)
 		if err != nil {
-			println(err)
-			continue
+			switch err {
+			case gps.ErrUnknownNMEASentence, gps.ErrInvalidNMEASentence, gps.ErrInvalidNMEASentenceLength:
+				continue
+			default:
+				println("parse error:", err)
+				continue
+			}
 		}
 		if fix.Valid {
 			print(fix.Time.Format("15:04:05"))
@@ -43,7 +52,7 @@ func main() {
 			}
 			println()
 		} else {
-			println("No fix")
+			println("Waiting for fix...")
 		}
 		time.Sleep(200 * time.Millisecond)
 	}