Builtin vs custom exceptions

Description of issue or feature request: Before releasing TUF 1.0.0 we should review our code and make sure that:

1. We are using built-in exceptions (ValueError, TypeError, KeyError, etc.) consistently and idiomatically.
2. We are using custom exceptions when it makes sense and not in places that could be replaced by a built-in exception.

_Initiated from comment https://github.com/theupdateframework/python-tuf/pull/1725#pullrequestreview-856729491_

[MVrachev]

We went through this with Martin and came back with this list:

• verify_signature() should handle SerializationError and throw UnsignedMetadata (this is what callers are interested in)
• verify_delegate() should document ValueError and TypeError
• from_securesystemslib_key should raise ValueError if securesystemslib.FormatError is caught
• All init() and from_dict() should document ValueError (and Keyerror if needed) – the reason this was not done is that this sort of makes it look like programmer should handle these errors… but as long as we are clear in the docstring that these are invalid argument errors, this should be fine

We will file issues for these so there is something actionable. After these issues are filed I believe this meta-issue can be closed

There are issues for all of those:

• #1816
• #1817
• #1818
• #1819

We can close this issue as it’s a little vague.

[jku]

We went through this with Martin and came back with this list:

• verify_signature() should handle SerializationError and throw UnsignedMetadata (this is what callers are interested in)
• verify_delegate() should document ValueError and TypeError
• from_securesystemslib_key should raise ValueError if securesystemslib.FormatError is caught
• All init() and from_dict() should document ValueError (and Keyerror if needed) – the reason this was not done is that this sort of makes it look like programmer should handle these errors… but as long as we are clear in the docstring that these are invalid argument errors, this should be fine

We will file issues for these so there is something actionable. After these issues are filed I believe this meta-issue can be closed