This updates RustfmtTool to subclass the newer BaseTool, and to use

the newer features it offers (dependencies, improved file extension

handling, and centralized base command building).
The new version of the tool also handles the case where a .rs file

can't be parsed due to a syntax error. Previously, we'd log the error

but the tool would claim it passed without issues. The new version

parses any error lines it finds and reports them as comments.

Unit tests pass on Python 2.7 and 3.x.