Exception hierarchy

[photonbit]

Is your feature request related to a problem? Please describe.

Inside cdsapi, all exceptions are instances of Exception, making the exception handling of the users of the library tricky, depending on making matches with the message of the exception.

Describe the solution you'd like

Instead, an exception hierarchy could be added to the project. Last week I have been working with cdsapi for a project and this week I might be collaborating this the main repo making PRs if it is alive to incorporate some improvements, like the exceptions hierarchy:

e5a88de

Describe alternatives you've considered

No response

Additional context

No response

Organisation

No response