Handle return code from BMA driver, and set a flag is the initialization fails. This allows to boot InfiniTime even if the device cannot initialize.

src/drivers/Bma421.h

@@ -25,12 +25,13 @@ namespace Pinetime {
       void Read(uint8_t registerAddress, uint8_t *buffer, size_t size);
       void Write(uint8_t registerAddress, const uint8_t *data, size_t size);
 
+      bool IsOk() const;
+
     private:
       TwiMaster& twiMaster;
       uint8_t deviceAddress = 0x18;
-
-
       struct bma4_dev bma;
+      bool isOk = false;
     };
   }
 }