svncommands: fix layering violation & tweak error messages. Importing `SubversionException' directly from `svn.core' is a layering violation: Anything within the Subversion bindings should only be accessed via svnwrap. The advantages to doing this are twofold: we only need to intercept missing bindings in one place, and we have the option of supporting alternate bindings. As an added bonus, the recently-added support for intercepting missing Subversion bindings actually works. Two error messages are tweaked to remove `It appears...' from one error message (just blaming Subversion instead) and make both errors include the URL in the suggested command line.