Improve fatal error messages

Signed-off-by: Michele Calgaro <michele.calgaro@yahoo.it>

flow/gsl/ConfigureChecks.cmake

@@ -42,13 +42,13 @@ if( WITH_VORBIS )
 
     else( VORBISFILE_FOUND )
 
-      tde_message_fatal( "Ogg/Vorbis support are requested, but `libvorbisfile` not found" )
+      tde_message_fatal( "Ogg/Vorbis was requested but `libvorbisfile` was not found on your system." )
 
     endif( VORBISFILE_FOUND )
 
   else( VORBIS_FOUND )
 
-    tde_message_fatal( "Ogg/Vorbis support are requested, but `libvorbis` but found" )
+    tde_message_fatal( "Ogg/Vorbis was requested but `libvorbis` was not found on your system." )
 
   endif( VORBIS_FOUND )
 
@@ -74,7 +74,7 @@ if( WITH_MAD )
     find_library( MAD_LIBRARIES NAMES mad )
     find_path( MAD_INCLUDE_DIRS mad.h )
     if( NOT MAD_LIBRARIES )
-        tde_message_fatal( "MAD support is requested, but `libmad` not found" )
+        tde_message_fatal( "MAD support was requested but `libmad` was not found on your system." )
     endif( NOT MAD_LIBRARIES )
   endif( MAD_FOUND )