Fix race condition in DiskCache trimmer causing DirectoryNotFoundException

[ronaldbarendse]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practise as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

This fixes issue https://github.com/JimBobSquarePants/ImageProcessor/issues/794 by catching exceptions thrown while enumerating files from directories that are deleted while the cache trimming is executed (e.g. by another server running a concurrent cache trimming job or manually deleting the directory).

[JimBobSquarePants]

Looks good! Thanks for picking this one up.

[ronaldbarendse]

Looks like RecursivelyDeleteEmptyDirectories() still throws DirectoryNotFoundExceptions, as the directory is deleted if it has no subdirectories/files and then tries to get its parent folder:

ImageProcessor/src/ImageProcessor.Web/Caching/DiskCache.cs

Lines 577 to 584 in 8498d14

	// If the directory is empty of files delete it to remove the FCN.
	if (Directory.GetFiles(directory, "*", SearchOption.TopDirectoryOnly).Length == 0
	&& Directory.GetDirectories(directory, "*", SearchOption.TopDirectoryOnly).Length == 0)
	{
	Directory.Delete(directory);
	}
	this.RecursivelyDeleteEmptyDirectories(Directory.GetParent(directory).FullName, root, token);

I think this whole method can be removed, as the cache trimmer already reverses all paths and therefore could do a single check (without recursively traversing to parent directories).