[Usability] Fix uncaught exception when unsupported labels are used in SDRF file

[jgriss]

Currently, the use of other label specifications causes the following error:

File "/opt/conda/envs/nf-core-proteomicstmt-1.0dev/lib/python3.8/site-packages/sdrf_pipelines/openms/openms.py", line 674, in save_search_settings_to_file
    URI + "\t" + raw + "\t" + f2c.file2mods[raw][0] + "\t" + f2c.file2mods[raw][1] + "\t" + label + "\t" +
UnboundLocalError: local variable 'label' referenced before assignment

Solution

Add proper error handling and well formatted (helpful) error message

(I will create a PR for this later on)

[ypriverol]

Solved.