Introduce custom exceptions

[jdemaeyer]

This introduces custom exception classes, all derived from click.ClickException. While there is currently no use for flow control, this has two main advantages:

• We can set default error messages for common errors, with the possibility to add default exit codes if we wish
• In tests, we no longer need to look for specific strings to check whether the correct error was raised

Also includes improvements in the config error messages (mainly when specifying undefined/invalid targets)