[utils] Improve swift_build_support.shell failure messages.

[ddunbar]

What's in this pull request?

[utils] Improve swift_build_support.shell failure messages.

 - This improves the error messages when commands fail (or don't exist) to show
   a one-line summary of the issue instead of the Python backtrace, and matches
   what was being done by the matching function in `SwiftBuildSupport`.

---

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• Test pull request on Swift continuous integration.

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[ddunbar]

(this depends on #2830, I will rebase once merged)

[ddunbar]

/cc @gribozavr

@swift-ci please Python lint

[gribozavr]

@swift-ci Please smoke test

[gribozavr]

OS X smoke test issue was unrelated, I removed the build directory.

@swift-ci Please smoke test OS X platform

[gribozavr]

@ddunbar

FAIL: Swift :: Python/swift_build_support.swift (2295 of 8267)
******************** TEST 'Swift :: Python/swift_build_support.swift' FAILED ********************
Script:
--
/usr/bin/python -m unittest discover -s /home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/swift/validation-test/Python/../../utils/swift_build_support
--
Exit Code: 1

Command Output (stderr):
--
ss....s.................................E................
======================================================================
ERROR: test_tar_nonexistent_file_raises (tests.test_tar.TarTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/swift/utils/swift_build_support/tests/test_tar.py", line 56, in test_tar_nonexistent_file_raises
    destination='/another/path/that/shouldnt/exist')
  File "/home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/swift/utils/swift_build_support/swift_build_support/tar.py", line 35, in tar
    shell.call(args + [source], stderr=subprocess.PIPE)
  File "/home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/swift/utils/swift_build_support/swift_build_support/shell.py", line 76, in call
    str(e.returncode) + ", aborting")
  File "/home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/swift/utils/swift_build_support/swift_build_support/diagnostics.py", line 32, in fatal
    raise SystemExit(sys.argv[0] + ": fatal error: " + message)
SystemExit: python -m unittest: fatal error: command terminated with a non-zero exit status 2, aborting

----------------------------------------------------------------------
Ran 57 tests in 8.371s

FAILED (errors=1, skipped=3)

--

[ddunbar]

Oops, didn't realize we had coverage of this. Will fix!

[ddunbar]

@swift-ci Please smoke test