Cleanup of sylvia/docs/API-UI-text

This is a rebased version of #255 with the following updates:

* Resolved merge conflicts with master.
* Squashed commits.
* Updated the unit test suite.
* Changed a few of the JavaScript server error messages.
* Changed single-quotes to double-quotes in the XML validation error messages. 

The original changes were:
* Review error messages and UI text
* Edit error messages in .py files
* Corrected tiny typo in models.py
* Revise UI and error message text in .js files
* Updated "Could not contact server" error messages

@srpearce If this looks good, we can merge this PR and close #255.  If not, we can do some git magic to get these changes onto your branch:
```
git checkout sylvia/docs/API-UI-text
git fetch
git reset --hard origin/will/cleanup-doc-branch
```
and when you're sure everything looks right, force-push your changes to your branch in GitHub.
```
git push -f
```
It is possible to mess things up horribly (like rewriting master) if your git settings aren't correct, so let me know if you need help with the git commands.