Add test for absence of file or string re #8546

Also correct the error message.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Add reference to ticket #8521 in comment re #8546

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Merge branch 'feature/8546_load_parameter_file_string' into develop