fix(reporter): keep users exact formatError result

[levithomason]

I wrote this feature originally and haven't had time to come back and address this issue until now. See #2119 (comment).

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Fixes #2626

The main purpose of the formatError function, per the docs, is to trim down the stack trace. However, no matter the return value configured, a new line is always appended. This means even when lines are removed a blank new line is still added. This results in large, blank, stack traces:

This PR simply returns the exact value the user has configured in their formatError function, without appending a newline to it. This allows for removing lines from the stack trace:

[maksimr]

@levithomason can you fix tests

Thanks

[maksimr]

@levithomason also it would be nice to write test on this problem

[levithomason]

Sure thing. This was an in browser patch. I'll come back and give a proper update.

[levithomason]

I've updated the failing test. I went to add an additional test, however, it will only duplicate the existing test as it will ensure no new line is added to the config.formatError output. Instead, I opted to change the test description to reflect the exact error result should be returned.

[maksimr]

LGTM

[levithomason]

Odd failures on CI. Can someone retrigger please?

Travis:
Just shows an infinite loading spinner for me...

AppVeyor failure

npm ERR! code EBADF
npm ERR! errno -4083
npm ERR! syscall fstat
npm ERR! EBADF: bad file descriptor, fstat
npm ERR! A complete log of this run can be found in:
npm ERR! C:\Users\appveyor\AppData\Roaming\npm-cache_logs\2017-03-27T05_29_49_919Z-debug.log
Command exited with code -4083

[levithomason]

I should note that npm test passes locally.

[levithomason]

@maksimr would you happen to have rights to retry tests on CI? Perhaps without cache if available?

[maksimr]

@levithomason only on travis and I have restarted it

[levithomason]

I appreciate it.

[dignifiedquire]

@levithomason can you rebase onto master please, then all linting issues should be fixed and ci hopefully happy

[levithomason]

Rebased and pushed. Lint and tests pass locally.

Travis looks like it will pass.

AppVeyor node 0.12

  1) launchers/process.js flow start -> timeout -> 3xrestart -> failure:
     Uncaught AssertionError: expected spy to have been called with arguments \usr\bin\browser, ["http://localhost/?id=fake-id"]
      at test\unit\launchers\process.spec.js:234:42
      at [object Object]._onTimeout (node_modules\lodash\index.js:1773:43)
  
Warning: Task "mochaTest:unit" failed.� Use --force to continue.
Aborted due to warnings.

[dignifiedquire]

I restarted appveyor lets see if that helps

[levithomason]

Looks like we're good to go here. Thanks for the restart.

[levithomason]

@dignifiedquire bump

[dignifiedquire]

Thank you

[donaldpipowitch]

Nice

[levithomason]

Awesome, thanks much. Can't wait to use this.