functionObject: Improved incorrect and incomplete argument error messages

Both the functionObject call context (the command line for postProcess, and the
controlDict path for run-time post-precessing) and the configuration file
context where the arguments are substituted are now printed in the error
message, e.g.

    postProcess -func 'patchAverage(name=inlet, ields=(p U))'

generates the message

--> FOAM FATAL IO ERROR:
Essential value for keyword 'fields' not set in function entry
    patchAverage(name=inlet, ields=(p U))
    in command line postProcess -func patchAverage(name=inlet, ields=(p U))
    Placeholder value is <field_names>

file: /home/dm2/henry/OpenFOAM/OpenFOAM-dev/etc/caseDicts/postProcessing/surfaceFieldValue/patchAverage from line 13 to line 17.

and with the following in controlDict

functions
{
    #includeFunc patchAverage(name=inlet, ields=(p U))
}

generates the message

--> FOAM FATAL IO ERROR:
Essential value for keyword 'fields' not set in function entry
     patchAverage(name=inlet, ields=(p U))
    in file /home/dm2/henry/OpenFOAM/OpenFOAM-dev/tutorials/incompressible/pimpleFoam/RAS/pitzDaily/system/controlDict at line 55
    Placeholder value is <field_names>

file: /home/dm2/henry/OpenFOAM/OpenFOAM-dev/etc/caseDicts/postProcessing/surfaceFieldValue/patchAverage from line 13 to line 17.

src/OpenFOAM/db/dictionary/functionEntries/functionEntry/functionEntry.H

@@ -78,8 +78,10 @@ public:
         //- Construct from keyword, parent dictionary and Istream
         functionEntry(const word&, const dictionary&, Istream&);
 
-        //- Disallow default bitwise copy construction
-        functionEntry(const functionEntry&) = delete;
+        autoPtr<entry> clone(const dictionary&) const
+        {
+            return autoPtr<entry>(new functionEntry(*this));
+        }
 
 
     // Member Function Selectors