When gwenview fails to create a thumbnail, it reuses the last displayed thumbnail.

Reproducible: Always

Steps to Reproduce:
1. Create a directory, put in it an image called a.jpg
2. Create an empty file called b.jpg  (for instance: touch b.jpg )
3. Launch gwenview on the directory

Actual Results:  
gwenview tries to create a thumbnail for b.jpg, but it is impossible as it is not a jpg file. However, gwenview does display a thumbnail for b.jpg: it reuses the one created for a.jpg. Then, the user sees twice the same picture with different names, which is confusing. The user might delete a.jpg believing there is a copy in b.jpg!

Expected Results:  
b.jpg should not be displayed by gwenview, or should be displayed without thumbnail, or with a "there was an error" thumbnail.

I have seen this bug after upgrading from fedora 21 to fedora 23, on a directory containing both .jpg and .tiff files. gwenview was unable to make thumbnails for the .tiff files because package qt5-qtimageformats was not installed (that was not obvious and took me some time to find out!) and so, silently, it reused for each tiff the latest successfully generated thumbnail (for a .jpg file). There was a moment of panic: were some files overwritten ?

I leave the severity as normal, although it could arguably be marked as critical, because this bug might induce the user to remove some files, believing there is a copy, and thus lose some data.