Fix freeze on non CKAN files in cache folder

[HebaruSan]

Problem

Some users reported that CKAN freezes while trying to load the registry at startup.

Cause

This exception is thrown:

System.ArgumentOutOfRangeException: 인덱스 및 길이는 문자열 내의 위치를 참조해야 합니다.
매개 변수 이름: length (Rough translation: The index and length must refer to the position within the string. Parameter name: length)
   위치: System.String.Substring(Int32 startIndex, Int32 length)
   위치: System.Linq.Lookup`2.Create[TSource](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   위치: System.Linq.GroupedEnumerable`3.GetEnumerator()
   위치: System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   위치: System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector)
   위치: CKAN.NetFileCache.GetCachedFilename(Uri url, Nullable`1 remoteTimestamp)

... which means that this block from #2563 tried to get a substring of a filename shorter than 8 characters:

CKAN/Core/Net/NetFileCache.cs

Lines 175 to 180 in 44d44ea

	cachedFiles = files = allFiles()
	.GroupBy(fi => fi.Name.Substring(0, 8))
	.ToDictionary(
	grp => grp.Key,
	grp => grp.First().FullName
	);

It appears that some users may have chosen a CKAN cache folder that is not exclusive to CKAN:

Changes

Now that block will ignore any filenames that don't start with 8 hex digits and a dash. This will both fix the problem with short filenames and exclude other files that were not created by CKAN even if they have long names.

Fixes #2711 (I think).

[DasSkelett]

Oh wow, didn't think of the possibility that there are other files with shorter names in the cache folder.

There's another issue happening:
Since v1.26, more precisely #2617, (fatal) errors are no longer thrown during startup. This is due to the whole process being a task now, and C# doesn't throw errors happening in tasks without using await.
This can be a big problem, because the only way to get a stack trace and error message is with a debugger, which is only an option if you know how to reproduce a bug. In #2711 this was a problem to, we only got the error text with a downgrade to v1.25.
I'll do a PR to fix this.

[HebaruSan]

Oh, oops. I assume we can keep the loading screen if we switch to async/await?

[DasSkelett]

Yes. It works exactly the same, but actually throws errors if they occur.
It's nothing more than a 2-line change, but see yourself in a few minutes, when my PR is done.

[HebaruSan]

OK, good. I've tried probably around five times now to learn async/await, and I still find threads much easier to understand.