More actionable error messages for setting up the Zencoder API module

I'm sharing a patch that will:

• provide error messages from FFMPEG or Zencoder abstract factory classes using drupal_set_message() to make them more visible to site builders.
• add link to module listing page when zencoderapi module is disabled
• describe how to download and install the Zencoder php library
• make the zencoderapi module less scary to enable on the module listing page

There's no "functionality" changes per se. Hopefully these changes will make it easier to get up and running for first time users that want to use Zencoder. Please consider accepting this patch to the 7.x-2.x branch! Thanks!!