`instance create` may return 403 for several different reasons

[Jon Perritt]

Currently we're catching a 403 and returning an error about the flavor or image not being suitable for the given parameters, but it turns out several different issues may return a 403 (for example, exceeding a RAM quota or exceeding the size limit of a personality file). We need to somehow figure out what to return based on the error message that Rackspace returns, which, unfortunately, probably means a regex on the error message.