adds 'url' to log context for rendering errors

[mattoberle]

When Amphora throws an error in a location where req/res context is available we don't currently include that context.

This commit includes url in the log context to make it easy to reproduce errors.

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According to the express docs res.locals is available by default and there is no risk of attempting to reference a property on an undefined object under normal use.

There may be more useful context available, but this seems like a good place to start.

For example, res.locals.user would log which user initiated an action (for internal requests using amphora-auth): https://github.com/clay/amphora-auth/blob/94201d60a6043c968b507a66e7ab289ce4f0139b/index.js#L144-L153

[coveralls]

Pull Request Test Coverage Report for Build 2463

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 2458:	0.0%
Covered Lines:	4197
Relevant Lines:	4197

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💛 - Coveralls

[mattoberle]

lgtm, but would also be a bit safer to use lodash _get(res, 'locals.url')

@reubenson yeah I was thinking about that too. It looks like Express sets res.locals so it should be there: https://expressjs.com/en/api.html#res.locals

The only situation I can think of where it wouldn't would be if someone added a middleware to wipe out that attribute or set it to a non-object (which seems like a mis-use of Express and maybe should result in a hard error?).

Edit: We already import lodash in these modules, might as well do the safer thing. It's probably worth keeping the side-effect code (ie. logging) as safe as possible.