APIError should make message more readily available to end users

[kwilsonmg]

Is there an existing issue for this?

• [X] I have searched the existing issues

Is your feature request related to a problem? Please describe.

Currently you can access the APIError's code but not the message that is also returned with it. This seems to have been an oversight.

Describe the solution you'd like.

A new property called "message" that returns the message, just like how the code is returned by the "code" property.

Describe an alternate solution.

N/A

Anything else? (Additional Context)

No response

[kwilsonmg]

Once this is merged, it makes solving #136 slightly easier as well...or at least fewer lines of code.