Replace log statements with informative str errs

Instead of using logging statements to provide context for errors in
Client.call I am now wrapping the error in a new errorString error. The
errors aren't really recoverable, so you can't gain much from inspecting
the real error anyway.

The new errors provide contextual information about the origin of the
error, so that the errors will make sense.

This is a breaking change for any code that would inspect the errors
previously returned.

go-mwclient no longer prints anything---all logging statements have been
removed.