Give more info when trying to build manual

Give more info when trying to build manual

The manual build would fail with cryptic reasons when Sphinx was not
available because the include was not properly set.

Also set up Sphinx detection to be verbose so that users will have it
easier to see whether they can build the docs or not.

Building the docs will not work now when using Python3.

Add test script to check if ImageMagick convert actually works or not.

Refs #2709

Change-Id: I1c49d683d971a5bf0b0ce217f7e9ecd60603e6e6