FailureListFormatter: pinpoint failure location and improve editor integration

[paddor]

Second attempt of #2839.

[paddor]

This second attempt tries to pinpoint the relevant source of the exception. Aggregated failures produce multiple lines in the output.

[paddor]

Fixed CI on Windows.

For more fine-grained matching in editors, I've included an error type (always E).

Before:

./foobar_spec.rb:12:expected false got true

After:

./foobar_spec.rb:12:E:expected false got true

Without this, it's tricky to safely distinguish these file:nr:msg lines from other lines. For example the simple errorformat='%f:%l:%m' in Vim would also recognize any line containing a timestamp as an error. Instead, errorformat='%f:%l:%t:%m' will only match lines like path:nr:type:msg.

This also allows for future extension of this formatter. For example, full backtraces could be printed as informational lines (I instead of E) if --backtrace was given.

[paddor]

@JonRowe @smortex

[paddor]

So after working with this for a day, I found myself running RSpec without -ff for a few times to see more of the backtrace. Thus, I've added support for --backtrace.

Users of this FailureListFormatter can configure their editors to either run RSpec with -ff or with -ff --backtrace to get informational (backtrace) lines along with the errors lines where the code failed. It's still the same format so capable editors can jump to the exact locations.

[paddor]

I've simplified it more. It always includes backtrace as informational lines. I find this the most useful output, making it very quick to follow how/where an spec failed.

Please review.

[pirj]

Closing as stale