Error when selecting certain folders in the import file browser

[yoda-vid]

@clifduhn reported that a crash can occur when opening the Import file browser and selecting certain folders. This appears to be related to trying to parse filenames in the folder.

[yoda-vid]

The crash occurs when selecting a folder to import, but the first file cannot be opened by skimage.io.imread. This may happen when a non-image file happens to be in a folder to be imported, but may also happen when inadvertently selecting a folder in the import file browser and then clicking on the open rather than the cancel button.

The fix is to catch image read exceptions, proceeding to the next file until an image is found. May want to filter files at this setup stage or provide more explicit warning regarding unloadable files since the import will likely fail in the next step.