Add warning if name not found in get_scanner_from_name

[robbietuk]

Changes in this pull request

Logs a warning message if scanner name is not present in the list of scanner name available.

Testing performed

Related issues

Fixes #1520

Checklist before requesting a review

• I have performed a self-review of my code
• [] I have added docstrings/doxygen in line with the guidance in the developer guide
• [] I have implemented unit tests that cover any new or modified functionality (if applicable)
• The code builds and runs on my machine
• [] documentation/release_XXX.md has been updated with any functionality change (if applicable)

[KrisThielemans]

ok. minimum change.

I guess really it should throw an error or so, but in that case, we'd have to catch it in various places presumably.

[robbietuk]

It should be an error but I don't know where/how return new Scanner(Unknown_scanner); is ever used. C++ string concatenation change is good. Feel free to squash merge.