Improve docstrings on exceptions

[MVrachev]

Current behavior:
Currently, in tuf/api/metadata.py we are using multiple ways to document our custom TUF exceptions.
There are two main ways of how we handle exceptions:

• there are places where we define an exception with the full module name:
  

  python-tuf/tuf/api/metadata.py

  Line 185 in 0f1fc6e

  tuf.api.serialization.DeserializationError:

• and in other cases, we document only the name of the exception:

  python-tuf/tuf/api/metadata.py

  Line 346 in 0f1fc6e

  UnsignedMetadataError: 'delegate' was not signed with required

Expected behavior:
Decide how we want to document our exceptions and stick to that.
Document how we should write docstrings about exceptions somewhere.
I didn't find any mentions about the exception docstrings in the Google Python guidelines

[MVrachev]

One solution I am proposing is using a hybrid of the two ways we document them now: document exception as
module_name.exception_name where module_name is the name we have given during import.
Example:

import tuf.api.exceptions as exceptions
...

def f():
"""
Raises:
  exceptions.UnsignedMetadataError: unsigned metadata
"""
...

[jku]

I would prefer no module path and just using the class name (this is what we do for other things -- see e.g. how we refer to TargetFile and not tuf.api.metadata.TargetFile in the ngclient documentation).

However, we probably should start publishing the exceptions documention on RTD so that there is a reference to look them up.

[jku]

I'll add the backlog tag with the hope that it leads to figuring out some better solution than the current worst-of-all-options (quite wordy in docstring, yet no linked docs on readthedocs)