Throw exceptions returned from BucketExistsAsync

[RoyalScribblz]

Fixes #1118

Throw exceptions returned in the response as BucketNotFoundException is already caught.

[michaelwv]

Any estimated date for releasing this bugfix?

[harshavardhana]

BucketExistsAsync must be returning false and no exceptions when bucket does not exist.

Please add a functional test to cover this scenario // cc @ebozduman

[ebozduman]

Checking the code and the existing test to see what is missing to capture the thrown exception as a failure...

[albracko]

Just to add here..all API methods should be checked not just this one, because Error Handling for other API methods is also wrong

[COCPORN]

I also ran into this. If the AccessKey or SecretKey is wrong, checking if a bucket exists returns true.

[ebozduman]

@RoyalScribblz , @michaelwv, @albracko , @COCPORN
I appreciate it if you could test and verify the fix, #1141 (not merged yet as of 07/24 23:42), against your scenarios.
If you are satisfied with your testing, you/I can close this PR.