Always reject getPixmap promise upon unexpected PS response

In some cases, if we receive an unexpected response from Photoshop to a `getPixmap` request, the promise is neither resolved nor rejected. If clients are pipelining requests, as in the Image Assets plugin, this can block their execution. This pull request ensures that the promise is rejected upon receiving an unexpected response.

Please review ASAP @joelrbrandt.

CC @timothynoel