Consistent Error Handling and minor bug fixes

Changes:

made error handling more consistent and always exit with a non 0 error code. https://github.com/bchr02/node-pre-gyp-github/issues/13
an error will now occur if NODE_PRE_GYP_GITHUB_TOKEN is not found.
slightly changed some of the error messages and removed some of the prepended "Error: ". They are no longer needed since were are now throwing errors.
fixed a bug that caused an error to not occur when binary files were not found in the stage directory.
added a missing error catch for readdir that caused an error to not occur if the stage directory was missing.
in bin/node-pre-gyp-github.js, corrected the path that was displayed in the help text.

bchr02 / node-pre-gyp-github