functionObjects::fieldExpression: Correct and improve warning messages

postProcess -func MachNo

previously generated the warning

Executing functionObjects
--> FOAM Warning :     functionObjects::MachNo MachNo cannot find required field U

which is incorrect; the field 'U' is available but the
thermophysicalProperties is not.  Now 'postProcess' generates the
warning:

Executing functionObjects
--> FOAM Warning :     functionObjects::MachNo MachNo cannot find required object thermophysicalProperties of type fluidThermo
--> FOAM Warning :     functionObjects::MachNo MachNo failed to execute.

Resolves bug-report http://bugs.openfoam.org/view.php?id=2352