ENH: additional method for switching error throwing on/off (issue #552)

- error::throwExceptions(bool) returning the previous state makes it
  easier to set and restore states.

- throwing() method to query the current handling (if required).

- the normal error::throwExceptions() and error::dontThrowExceptions()
  also return the previous state, to make it easier to restore later.

applications/utilities/postProcessing/postProcess/postProcess.C

@@ -191,7 +191,7 @@ int main(int argc, char *argv[])
             );
         }
 
-        FatalIOError.throwExceptions();
+        const bool throwingIOErr = FatalIOError.throwExceptions();
 
         try
         {
@@ -208,12 +208,15 @@ int main(int argc, char *argv[])
             // Report to output (avoid overwriting values from simulation)
             profiling::print(Info);
         }
-        catch (IOerror& err)
+        catch (Foam::IOerror& err)
         {
             Warning<< err << endl;
         }
 
         Info<< endl;
+
+        // Restore previous exception throwing state
+        FatalIOError.throwExceptions(throwingIOErr);
     }
 
     Info<< "End\n" << endl;