Retry failed web requests, fix install stanza error

[HebaruSan]

Problems

• Since Multi-find fixes #3074, a mod with a bad install stanza says, "Sequence contains no elements" instead of explaining that the stanza didn't match any files
• Even if the previous point is fixed, a mod using the default install stanza will throw a null reference error if it doesn't match any files
• Warnings about fetching remote version files don't print the remote URL (which would have been convenient for the following point)
• Prior to the recent flood of SpaceDock "The request was canceled" spam, after Purge CurlSharp #3118, the netkan-bot channel was plagued with these errors:
  
  This is mostly harmless, unless a mod author is actually using the remote version file to update compatibility after release, in which case it can cause CKAN's metadata to temporarily revert to the at-release state.

Causes

• The code to find shortestMatch threw an exception if there were no matches, when it should have defaulted to null
• CkanModule.DescribeInstallStanzas assumed install would not be null
• ksp-avc.cybutek.net and ksp.spacetux.net are flaky servers, probably because they're run by individuals rather than big companies with cloud resources. They seem to fail in about 0.1% of requests; prior to Purge CurlSharp #3118, we fell back to CurlSharp in these cases, but after that PR the fallback is removed.

Changes

• Now errors like Module contains no files matching: find="Cosmogator" are back
• Now CkanModule.DescribeInstallStanzas knows how default install stanzas work
• Now remote version file fetch errors print the URL
• Now if a URL fails for non-protocol reasons (i.e., if it's a network failure rather than a HTTP 404), we try again immediately, then after waiting 0.1s, then 0.2s, then 0.3s. If it still doesn't work, we finally give up and fail as before.

[DasSkelett]

Nice, thanks! Can't wait for the bot to stop spamming all the download errors.

[HebaruSan]

Yeah, I may un-mute the netkan-bot channel soon!