Integrate Whoops error handler.

[demiankatz]

Whoops is an advanced PHP error handler which provides a lot of useful information to help with debugging and a user-friendly interface for inspecting stack traces. This PR integrates it with VuFind, activating it only when development mode is turned on.

TODO

• Find out whether Whoops can be configured to display previous exceptions. By default, it only appears to display the most recent exception, which makes troubleshooting things like ServiceManager failures nearly impossible. EDIT: the full exception stack is there; it's just a bit hard to differentiate between exceptions and function calls.
• Configure Whoops to allow appropriate VuFind exceptions (like "missing record") to pass through without being displayed.
• Investigate other Whoops configuration options and determine the best way to configure Whoops within VuFind's infrastructure.
• Talk to the maintainer of ghislainf/zf2-whoops and see if an official release can be minted to allow us to pin a version. Could not get an official 1.0 release minted on the zf2-whoops project; have pinned a specific commit for now. Opened an issue requesting the release (see issue Can we have a 1.0 release? ghislainf/zf2-whoops#21); will fork the project in the future if major changes are ever needed.
• Wait until VuFind's version requirement has been raised to at least 5.5.9 before merging due to dependencies.

[demiankatz]

I have made some progress on this -- the module is now configured to allow "RecordMissing" exceptions to pass through, and it appears that it does correctly represent the entire exception stack -- I was just failing to read the output correctly when I last looked at it.

I think the main outstanding issue is getting a 1.0.0 release of ghislainf/zf2-whoops so that we can pin a version in our composer.json; I have written to the lead developer of the project to see if he is willing and able to help with that.

[demiankatz]

It's been nearly a month, and no reply from the lead developer, so I have tried contacting Balázs Németh, another person listed as an author in the project's composer.json (and the only one with an email address listed).

[demiankatz]

Balázs Németh responded promptly but unfortunately is not actually a maintainer of the package (he is the author of an earlier version that was adapted into the current form). I notice that ghislainf has not had any GitHub activity since January, so perhaps he is away at the moment. If I notice activity on his account, I will try sending a reminder email.

[demiankatz]

Another month has passed without progress, so I have opened an issue on the project's GitHub account requesting a release and offering help if necessary.