Correctly handle failed pip/setuptools/wheel installs (#1007)

They are now displayed in the build output (instead of being sent to
`/dev/null`) and fail the build early instead of failing later in
`bin/steps/pip-install`.

Fixes #1002.

CHANGELOG.md

@@ -5,6 +5,7 @@
 - For repeat builds, also manage the installed versions of setuptools/wheel, rather than just that of pip (#1007).
 - Install an explicit version of wheel rather than the latest release at the time (#1007).
 - Output the installed version of pip, setuptools and wheel in the build log (#1007).
+- Errors installing pip/setuptools/wheel are now displayed in the build output and fail the build early (#1007).
 - Install pip using itself rather than `get-pip.py` (#1007).
 - Disable pip's version check + cache when installing pip/setuptools/wheel (#1007).
 - Install setuptools from PyPI rather than a vendored copy (#1007).

bin/steps/python

@@ -135,6 +135,8 @@ if [ ! "$SKIP_INSTALL" ]; then
   hash -r
 fi
 
+set -e
+
 PIP_VERSION='20.0.2'
 SETUPTOOLS_VERSION='39.0.1'
 WHEEL_VERSION='0.34.2'
@@ -174,8 +176,7 @@ fi
 
 puts-step "Installing pip ${PIP_VERSION}, setuptools ${SETUPTOOLS_VERSION} and wheel ${WHEEL_VERSION}"
 
-/app/.heroku/python/bin/python "${PIP_WHEEL}/pip" install --disable-pip-version-check --no-cache \
-  "${PIP_TO_INSTALL}" "setuptools==${SETUPTOOLS_VERSION}" "wheel==${WHEEL_VERSION}" &> /dev/null
+/app/.heroku/python/bin/python "${PIP_WHEEL}/pip" install --quiet --disable-pip-version-check --no-cache \
+  "${PIP_TO_INSTALL}" "setuptools==${SETUPTOOLS_VERSION}" "wheel==${WHEEL_VERSION}"
 
-set -e
 hash -r