Add a count of errors to exception message in XeroBadRequest.
Add a count of errors to exception message in XeroBadRequest.

If there are multiple errors in a HTTP 400 response, we include the
first one in the main message, and also mention how many other
errors there are.

freakboy3742/pyxero,jarekwg/pyxero,direvus/pyxero,schinckel/pyxero,wegotpop/pyxero