Improving the error messages when TPL find fails (TriBITS #41)

This is the first step in improving the info for configuration TPLs and the
error messages when a TPL find fails.  Currently, I am only testing a
header-ony TPL but that behavior looks good.

A few things I did as part of this:

* The processing of TPLs now starts with "Processing enabled TPL" instead of
  "-- Processing enabled TPL" and I reduced the indent of the "--" lines by
  two spaces.

* It now shows what package eanble the TPL and makes it very clear how to
  disable the TPL and what impact that will have.

* The CMake processing no longer aborts with FATAL_ERROR inside of the
  TRIBITS_TPL_DECLARE_LIBRARIES() function.  Instead, it sets a global var
  TPL_${TPL_NAME}_NOT_FOUND=ON and does a RETURN().  Then, the calling TriBITS
  code checks this and then prints more info and then aborts.