GattStatusException should include the GATT status

[joharei]

I would like to handle errors based on the GATT status, but the GattStatusException doesn't expose that. Currently, I can work around this by checking the exception message, but a nicer solution would be great!

[twyatt]

Totally agree, checking GATT status via the message is quite fragile.

Need to determine if there is an elegant way to expose the status in a platform agnostic way.
In other words, need to determine if the GATT statuses (which represent non-success) have enough overlap between the platforms to try and translate the platform specific statuses to a common structure (e.g. enum or similar).

Currently, the exceptions are propagated as:

• Android: GattStatusException
• Apple: IOException (with NSError description as message)
• JavaScript: native failure type (from Promise)

...so this is already exposing inconsistency between the supported platforms. When I have some time I'll see if we can come up with a better API.

[twyatt]

Related to #724.