- Previously we used message priority: SEND_ERROR when a package was
  not found and find_package() was called without QUIET or REQUIRED,
  which emits an error message, and prevents generation, but continues
  configuration.
- The fact SEND_ERROR induces an error message was confusing for users
  as it implies that something bad has happened and they cannot
  continue, when in fact we were disabling the option in question
  and were thus able to continue, all they had to do was re-configure.

- This commit also reorders the search lists for includes/libraries
  so that we always search user installed locations (e.g. /usr/local)
  before system installed locations.  Thus we will now always prefer
  a user install to a system install if both are available, which is
  likely to be the users desired intention.

Change-Id: Ide84919f27d3373f31282f70c685720cd77a6723