Provide a more helpful error if the instrument definition file is not found

[Russell Taylor]

Right now, if the IDF is missing and you try to load a file for that instrument you see this error message:

Error in execution of algorithm LoadInstrument:
SAXParseException: No element found in line 1 column 0. Unable to parse XML in 
Error loading Instrument definition file

Clearly, there is no check on whether the file was successfully located or not.

[Russell Taylor]

The commit didn't appear here by itself:

Re #8106. Check for failure to find an IDF in LoadInstrument

…and provide an error message in that case. Previously, it would plough on into the parser and you'd get some SAX error which provided few clues to the real failure reason.

b016af25d89fef11dde81f0f07df313798d2f606

[Russell Taylor]

To test, try deleting the instrument definition file(s) for a particular instrument and then loading a file from that instrument. You should see a message informing you that the file was not found, instead of the error shown in the Description above.

[Michael Reuter]

Looks like the automatic closer missed this one. Putting the changeset in for recordkeeping.

​https://github.com/mantidproject/mantid/commit/b9c89c36d8ad0ba34568479d8eff9d80108bf6b1

[Stuart Campbell]

This ticket has been transferred to github issue ​8951