test: better message for test-child-process-emfile failures

[Trott]

When the test fails (as it does frequently on FreeBSD unfortunately)
provide a non-cryptic error message that also provides a meaningful line
number (test-child-process-emfile.js:33).

Before:

#assert.js:87
#  throw new assert.AssertionError({
#  ^
#AssertionError: 0 undefined null
#    at emitTwo (events.js:87:13)
#    at ChildProcess.emit (events.js:172:7)
#    at Process.ChildProcess._handle.onexit (internal/child_process.js:200:12)

After:

#assert.js:89
#  throw new assert.AssertionError({
#  ^
#AssertionError: "exit" should not be emitted (the process never spawned!)
#    at ChildProcess.<anonymous> (/usr/home/iojs/build/workspace/node-test-commit-other/nodes/freebsd101-64/test/sequential/test-child-process-emfile.js:33:10)
#    at emitTwo (events.js:87:13)
#    at ChildProcess.emit (events.js:172:7)
#    at Process.ChildProcess._handle.onexit (internal/child_process.js:200:12)

[Trott]

CI: https://ci.nodejs.org/job/node-test-commit/811/

[bnoordhuis]

LGTM but the subject line is > 50 characters.

[Trott]

Squashed commits, fixed > 50 char commit log line

[Trott]

Landed in e3d9d25

[MylesBorins]

consideration for LTS?
@jasnell

[jasnell]

Landed in v4.x-staging in e43c611