Bugfix 0x1003

[jhunkeler]

This PR addresses:

• Error output from pip and conda while testing if a package exists on an index (or not) was too verbose. This reduces unnecessary output by only triggering the process output dump when a "real" error occurs.
• Test blocks with empty scripts, or errors, did not return (popd) from the top-level pushd
• Errant linefeeds in multiple SYS*() macro format strings were removed
• When shell()'s forked process is killed by a signal the return value should be non-zero