Adapt mocks a bit so that they behave more like NGitLab

[louis-z]

• Get rid of Mocks-specific GitLabBadRequestException, GitLabForbiddenException and GitLabNotFoundException
• Use NGitLab's GitLabException instead
• Add internal static methods to GitLabException, to ease the creation of specific exceptions

Closes to #841

[Toa741]

• NotFound exception is only in mock
• Can be misleading (catching this type only instead of the base exception)
• Remove GitLabNotFoundException, use ctors or methods to create GitLabException with correct status code, so the mock & client always return the same type of exception