Improve error message for partner validation

[oeway]

I got an error: "Expected dict type for loaded source, but got: <class 'str'>" for the partner validation CI but no idea where does the error come from: https://github.com/bioimage-io/bioimage-io-resources/runs/6385884221?check_suite_focus=true#step:4:24

Edit: Same here: https://github.com/imjoy-team/bioimage-io-resources/runs/6385989752?check_suite_focus=true#step:4:24

cc @FynnBe

[FynnBe]

the file path is given as string. the file does not exist though so it is not recognized as a file path and instead loaded as a yaml string. the file name string loaded as yaml remains a string.... thus the 'loaded' rdf is a string now...
I improved the error message in #454
fixed the validation in the imjoy repo here: imjoy-team/bioimage-io-resources#11
and fixed the validation in bioimage-io-resources here: bioimage-io/bioimage-io-resources@5395274

[oeway]

Got it! Thanks!