Add client exceptions

[llucax]

Add a ApiClientError exception that is the base class for all client exceptions. This exception is raised when there is an error in the client. We also wrap all gRPC response errors in a GrpcError exception that is a subclass of ApiClientError, and add one subclass for each possible gRPC status code.

This makes error handling more pythonic, as one can now just catch the exception type one is interested in, without having to do a second-level matching using the status.

It also helps avoiding to expose the grpclib classes to the user.

[llucax]

This is a port of:

• Add specific gRPC client errors frequenz-client-microgrid-python#53

With the following changes:

• Renamed ClientError to ApiClientError to make it less ambiguous in contexts where there might be other types of clients (like a customer).
• Made it compatible with grpcio too, at the expense of type checks being deferred to runtime. This should be temporary though, until we can battle-test betterproto and decide if we do the switch or stay with grpcio. I tried a type-safe approach using generics but it was too intrusive, as we would have to explicitly mention the underlying grpc error type when using every exception, like OperationTimedOut[GrpclibError].
• Added 2 properties to GrpcError: grpclib_error and gprcio_error to make it easier to access the underlying error for a particular library
• Added a list of subclasses in the docs, so it is easier to have a quick look at every possible gRPC error one could encounter.