Our Subversion support split the responsibility for error handling up

into SVNTool.normalize_error() and into individual backend functions.

The SVNTool version was even checking for backend-specific errors, a

holdover from when backends were first introduced.
In general, error handling for Subversion is iffy, as it depends on

backend internals. libsvn and PySVN may produce error strings that are

all wrapped by a single exception, which may be used for backend errors,

PySVN configuration errors, or repository errors. If operating inside a

callback, we also can't raise our own exceptions (which will come up and

be addressed in a future change).
This change redoes all our error and operation handling. Now, SVNTool

leaves all error handling up to the client backends. Backends are

expected to invoke operations within a with self.communicate(): block,

which will capture any errors and then hand them to the backend to turn

into proper exceptions. This will also give us an avenue to raise our

own exceptions within callbacks later, for SSL.
PySVN backend communication code has been updated to work with

communicate(). _do_on_path() now better wraps this as a context

manager as well, simplifying various backend implementations and letting

us remove some private callback functions.

All unit tests pass.