fix(gotrue): Handle empty error response

[Vinzent03]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

When receiving an error response, which doesn't contain any body, the decoding to json throws a FormatException. This is very misleading, because we shouldn't try to decode an empty response. This happened to me while doing some testing, but I do not exactly remember the reason for the error anymore.

What is the new behavior?

Do not decode an empty response.

In addition, I added a toString for every subclass of AuthException.

Additional context

[coveralls]

Pull Request Test Coverage Report for Build 14179551608

Details

• 17 of 27 (62.96%) changed or added relevant lines in 4 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 75.252%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/gotrue/lib/src/types/auth_exception.dart	4	14	28.57%

Files with Coverage Reduction	New Missed Lines	%
packages/gotrue/lib/src/gotrue_client.dart	1	64.99%
packages/gotrue/lib/src/fetch.dart	2	92.59%

Totals
Change from base Build 14080727348:	-0.2%
Covered Lines:	2840
Relevant Lines:	3774

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💛 - Coveralls

[dshukertjr]

Nice! Thanks for this!