Refactoring in preparation for follow up changes.
Also:
* Adds a new AssertErrorMessageEquals() helper method.
* Improved error reporting in the accessor and added name to
extractorProperty to ensure meaningful errors are reported.
* Added String() string method to propertiesContainer.
* Reports errors using the field name as the errors are not really
fixable by developers and it is more meaningful to the build team.
Bug: 155628860
Test: m nothing
Change-Id: I5c5b8436bcbc39e4e7cd35df2577b2dac53e702a