Inconsistent exception classes used across "ClientInterface" implementations

[aik099]

The CurlClient is:

• throwing \Exception when unsupported AuthenticationInterface implementation given
• throwing \chobie\Jira\Api\UnauthorizedException exception, when user can't be authorized
• throwing \chobie\Jira\Api\Exception in other cases

The PHPClient however is throwing \Exception in all cases, which makes it impossible to write code (that uses this library), that would catch specific exception types and would continue to work when client of the library would be switched from CurlClient to the PHPClient.

In particular this break logic of Walker::valid method, because it catches \chobie\Jira\Api\UnauthorizedException exception specifically.

[aik099]

Fixed in #108.