Revise FindQwt.cmake.

Revise FindQwt.cmake to fix the following:
- in case of error, don't set version="0.0.0" to provoke upstream
  fatal error, but exit immediately with clear error message;
- search qwt.h in all possible combinations of include paths
  and qwt subdirectory names, using CMake's PATH_SUFFIXES parameter;
- try versioned directory and library names first.

refs #9619
Merge branch 'upstream/bugfix/fix_findqwt_error_report' of github.com:jcns/mantid into jcns-upstream/bugfix/fix_findqwt_error_report

Conflicts:
	Code/Mantid/Build/CMake/FindQwt.cmake
Merge branch 'jcns-upstream/bugfix/fix_findqwt_error_report' into bugfix/fix_findqwt_error_report
Merge branch 'bugfix/fix_findqwt_error_report' into develop