improve error when an object fails to serialize

[braingram]

Description

asdf currently produces an unhelpful message for some objects that fail to serialize during asdf.write_to:

import asdf, collections

class MyDict(collections.UserDict):
    pass

asdf.AsdfFile({"d": MyDict({"a": 1})}).write_to("foo.asdf")

Produces:

...
yaml.representer.RepresenterError: ('cannot represent an object', {'a': 1})

This PR adds a new exception AsdfSerializationError that includes the object type in the error message when yaml emits a yaml.representer.RepresenterError (for backwards compatibility the new exception inherits from yaml.representer.RepresenterError).

Running the above example with this PR produces:

asdf.exceptions.AsdfSerializationError: ("Object of type[<class '__main__.MyDict'>] is not serializable by asdf. Please convert the object to a supported type or implement a Converter for this type to allow the tree to be serialized.", {'a': 1})

Here is a related issue where the above message would have been helpful: spacetelescope/roman_datamodels#361

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[zacharyburnett]

self-documenting errors!