ignore PotentialDataLoss

When the same circumstances as in [this Twisted ticket](http://twistedmatrix.com/trac/ticket/4840) occur, i.e. a server that sends no content-length or transfer-encoding header, an error is fired when attempting to read the content of a response. This patch ignores the `PotentialDataLoss` exception because in the majority of cases the exception is raised even on succesfull receiving of the response content. More fine-grained error handling is probably not possible at the moment due to the Twisted issue I mentioned earlier.