Improve exceptions

[Art4]

At the moment I have the problem that exceptions thrown by the library cannot be distinguished from other exceptions thrown for other reasons. This makes testing with the library a bit harder.

At the moment we throw only two different exceptions:

1. \Exception, when something fails during the transfer to Redmine or if an API parameter is invalid or missing
2. \InvalidArgumentException when a non-existent API is requested from the client.

This PR introduces these new exceptions that extend the previous exceptions:

1. \Redmine\Exception\ClientException replaces \Exception in CurlClient and Psr18Client.
2. \Redmine\Exception\InvalidApiNameException replaces \InvalidArgumentException('... is not a valid api. Possible apis are ...').
3. \Redmine\Exception\MissingParameterException replaces \Exception('Missing mandatory parameters').
4. \Redmine\Exception\InvalidParameterException replaces \Exception('Possible values for ...').

Additionally these new exceptions implement the interface \Redmine\Exception.

This makes it easier to catch only the exceptions thrown by the Redmine library. Nevertheless, we preserve backward compatibility.

try {
    $client->getApi('issues')->create($data);
} catch (\Redmine\Exception $e) {
    // exceptions from kbsali/redmine-api
} catch (\Throwable $e) {
    // other exceptions
}

What do you think, @kbsali? If you like the idea, I will create new tests for the new exceptions.

[kbsali]

I love the idea! 👍

[Art4]

I love the idea! 👍

Thank you. I will finalize the PR.

Btw what do you think about replacing travic-ci with Github Actions?

[Art4]

I've found two more cases where exceptions can be thrown and added two new Exception: MissingParameterException and InvalidParameterException.

[kbsali]

👍

[Art4]

This PR is ready for review. Please take a special look at the wording in the docs. 😊

[kbsali]

very nice! Thanks for this big PR and sorry for the late review.
Only minor comments, you could decide not to address them! :)