Improve exception hierarchy

[trishankatdatadog]

Description of issue or feature request:

Improve the exception hierarchy in TUF.

Current behavior:

Right now, every exception in TUF is a NoWorkingMirrorError(), which makes for very misleading error messages, to say the least, especially for front-end users.

Expected behavior:

Every exception should say exactly what the real error was.

[lukpueh]

Slightly related bad error message: lukpueh@e3e39cf

[jku]

I think we are far better about it now:

• there are fewer errors: only ones that we handle or that we actually expect clients (or future repository tools) to handle

• there is a hierarchy:

  • The different RepositoryErrors are useful but for a client they all mean "something is wrong on the repository side metadata and the issue is something we can't fix: A refresh later may help"
  • There are multiple DownloadErrors which for a client are all the same: "something goes wrong during download. trying again later might work"

  Closing, please open a new one if there are still issues