Enable pipefail for proper error propagation

[apjanke]

Fixes #216.

This sets the bash pipefail option, which causes the exit status of the pipeline to be the latest non-success status, instead of just the last element's status. This way, failures in the tar command will abort the build with a failure status.

[catch22]

This look good. However, it reveals another problem, namely that even when the tar call fails (imagine macOS without gtar installed) it will create an empty tar file, so another invocation of make thinks that this target has been built successfully:

$ make dist
mkdir -p "tmp"
Creating package version 0.7.0 release ...
rm -f -r "tmp/doctest-0.7.0"
git archive --format=tar --prefix="tmp/doctest-0.7.0/" HEAD | tar  -x
rm -f "tmp/doctest-0.7.0/README.matlab.md" \
	      "tmp/doctest-0.7.0/.gitignore" \
	      "tmp/doctest-0.7.0/.travis.yml" \
	      "tmp/doctest-0.7.0/.mailmap"
rm -f -r "tmp/doctest-0.7.0/util"
chmod -R a+rX,u+w,go-w "tmp/doctest-0.7.0"
tar: Option --mtime=2019-04-11 04:13:25 -0400 is not supported
Usage:
  List:    tar -tf <archive-filename>
  Extract: tar -xf <archive-filename>
  Create:  tar -cf <archive-filename> [filenames...]
  Help:    tar --help

$ make dist
make: Nothing to be done for `dist'.

Should we delete the tar file when the pipe fails?

[apjanke]

Should we delete the tar file when the pipe fails?

Yeah, but I'm not sure how to do that here.

[cbm755]

None-the-less, this is a step forward, merging..., then I'll reopen the issue and link here.