Refs #1419 - Changing a log message priority from error to debug.

In the case where a user wishes to load a file with a name which is not of the
"standard" format (including a valid instrument), our call to the
ConfigService getInstrument method should not report an error.

A debug message is sufficient, and there is still an exception thrown which
a class user can then interpret in any way he/she sees fit.

Code/Mantid/Framework/Kernel/src/ConfigService.cpp

@@ -1598,7 +1598,7 @@ const InstrumentInfo & ConfigServiceImpl::getInstrument(const std::string& instr
       // Move along, there's nothing to see here...
     }
   }
-  g_log.error("Instrument " + instrumentName + " not found");
+  g_log.debug("Instrument " + instrumentName + " not found");
   throw Exception::NotFoundError("Instrument", instrumentName);
 }