indexing context: make it easier to log and report on indexing status

[rjbs]

This branch provides a common interface (the context object) for collecting and reporting on errors or warnings found during indexing. It also replaces the "set skip or return early" logic with an exception system, to allow deep-down checks to declare that a bit of indexing can't succeed.

I have compared the messages generated by test runs, and believe that while there are some changes, they are either neutral or improvements.

Here's more detail:

• PAUSE::Indexer::Abort::Dist and PAUSE::Indexer::Abort::Package are introduced as exception types
• PAUSE::Indexer::Context is added. We create one of these objects for each dist we index. This object tracks errors and warnings during indexing. It tracks whether packages have been indexed or not. It also can generate the exceptions above to abort indexing in progress.
• PAUSE::Indexer::Errors is a registry of known errors that can happen. If we want to have a new kind of error, we add it here. We use that registry to provide the short and long form. This means that every error is now reported with both forms, whereas before some only had one.
• Almost everything else is built on using the changes above. Previously, flags were set on pmfile or dists indicating problems. Later, something would check those flags. All the code that worked this way has been updated to signal an abort. The index_packages and maybe_index_dist methods have been updated to catch abort exceptions and handle them.

[wolfsage]

A bunch of code can also be removed if you wish like things referencing INDEX_WARNINGS and has_index_warnings... etc

[rjbs]

I reworded the awkward commit. I also removed INDEX_WARNING stuff, which led me to find that the prelude-to-warnings text was not being included.

[rjbs]

@wolfsage As discussed today, I think this is reviewable.

[rjbs]

I've added commits from PAUSE::Indexer::Errors: fix keys from $old to PAUSE::dist: do not include private errors in report which I think address the last(???) outstanding thing. I'm going to look into coverage a bit more.

[rjbs]

I added code to let me get an mbox of all mail sent during testing, which I then inspected in my MUA. Looks pretty good.

[rjbs]

I added just a bit more test coverage for a notable but never-called-during-tests method.