Bash and its descendants allow having $PATH entries containing literal tildes,
like:
  ...:~/depot_tools:...
If the user has their shell configured this way, their shell will be able to
invoke `fetch` and other depot_tools commands, but depot_tools will not in turn
be able to invoke subcommands via popen because popen does not have this
bash-specific behavior when not in shell mode.

This change has gclient detect this configuration problem and error out with a
descriptive error message. Not doing so leads to a puzzling condition where
gclient complains about being unable to find a program that (from the user's
perspective) is in their $PATH and invokable interactively.

It also has 'fetch' suppress its own failure stack trace if it runs gclient and
gclient fails; the 'fetch' stack trace pushes the messages from gclient (which
are actually diagnostic) off the screen.

Bug: chromium:952865
Change-Id: Ibba4d2fccee405aa68392ce141493f1de21ec018
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1573204
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>