feat: Increase coverage.

[terjekv]

Eventually fixes #483

First pass:

• Add pragma to exempt lines from coverage to .coveragerc
• Add test for stringifying HostPolicyComponent-based objects (see https://github.com/unioslo/mreg/issues/482)
• Mock host_permissions/user_in_required_groups as its not supported by default.
• Ensure that IsSuperOrAdminOrReadOnly is tested for unauthenticated users (test_permissions/test_get_at_different_privilege_levels)
• Exclude exceptions from coverage that are impossible to reach without creating a broken API.
• Test that IsSuperOrNetworkAdminMember returns 401 for unauthenticated users (test_networks/test_networks_get_401_unauthorized).

[coveralls]

Coverage: 98.864% (+0.6%) from 98.286% when pulling 42d2e44bd9e48f0f1c7114543ec3c454a80132f2 on terjekv:terjekv/issue483 into b09877b2951f740727c550f65259298429ca3bc5 on unioslo:master.

[terjekv]

The next steps is to test https://github.com/unioslo/mreg/blob/master/mreg/signals.py and https://github.com/unioslo/mreg/blob/master/mreg/mqsender.py. There is no need to take that work into this PR. Opening for review.