Throwing GravatarException on noncatching service methods

[hamorillo]

Closes #377

Description

We want to wrap all exceptions that can occur inside the Gravatar service methods in our own GravatarException to provide the ErrorType as we do in the catching variants of those methods.

With this approach, we'll provide detailed information about the error, but at the same time, third-party developers know what exceptions can be expected from the Gravatar Services, basically GravatarException.

Testing Steps

• Code review (including test)
• Smoke test demo-app (although we are not using nonCatching method there)

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar Demo by scanning the QR code below to install the corresponding build.

	App Name	Gravatar Demo
	Commit	1976a412314c358c30ce325418c1153677a40776
	Direct Download	gravatar-demo-prototype-build-pr392-1976a41.apk