Fix typo that breaks wsidicom when "handles" is not present in config file

[CGDogan]

Change 1: Fix typo that broke wsidicom

Otherwise it says not found in the map in wsiimagehandle.py:

WSI_HANDLES = {
    "openslide" : "histoqc.wsihandles.OpenSlideHandle",
    "wsidicom" : "histoqc.wsihandles.DicomHandle"
}

Change 2:

Gracefully handle inexistant handles

It used to fail on the next line and break the loop and not try the remaining handles

[nanli-emory]

Hi @CGDogan, good catch. ;)