Improve API response to sendTransaction calls

[arajasek]

Description

The API currently doesn't provide very meaningful responses when sendTransaction() is called. This PR improves the responses it send back. Note that lots of the functionality provided in this PR isn't currently used, but might be helpful in the future. For example, whether a transaction is successfully sent, or already sealed, or repaid, the API still just responds with the hash in all cases. However, this PR lets the API distinguish between these cases, so in the future it would be easy to let the user know what happened.

Type of change

Insert x into the following checkboxes to confirm (eg. [x]):

• Bug fix.
• New feature.
• Enhancement.
• Unit test.
• Breaking change (a fix or feature that causes existing functionality to not work as expected).
• Requires documentation update.

Testing

Modified the existing modApiServer sendTransaction test. Testing of the PendingState changes is desirable, but hard to implement, so it's not included in this PR.

Verification

Insert x into the following checkboxes to confirm (eg. [x]):

• I have self-reviewed my own code and conformed to the style guidelines of this project.
• New and existing tests pass locally with my changes.
• I have added tests for my fix or feature.
• I have made appropriate changes to the corresponding documentation.
• My code generates no new warnings.
• Any dependent changes have been made.