Add error code and category funcs

[jake-engelberg]

Description

In order to support error code metrics, sometimes the category will need to be passed along with a kit error, as well as the ability to retrieve it and its error code.

Issue reference

Please reference the issue this PR will close: dapr/proposals#67

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests

[elena-kolevska]

Can you please add a unit test for ErrorCode() ?
Also, if I remember correctly, we should be checking if the tag field has a value first, before checking ErrorInfo.

[jake-engelberg]

Can you please add a unit test for ErrorCode() ? Also, if I remember correctly, we should be checking if the tag field has a value first, before checking ErrorInfo.

Added testing for ErrorCode().

Also, I've added the prioritization of tag in the ErrorCode() method itself rather than the ConvertErrorDetails() method, if you think I should move it in there lmk.

[elena-kolevska]

Great work!
We ended up doing the check for the legacy tag twice (you had it right the first time), but that's ok, I don't want to delay this PR.