baur run: execution is not aborted when running a task failed

[fho]

Example:

$ ../baur/baur run rng.build unixtime.build
Evaluating status of tasks:

rng.build          => Pending
unixtime.build     => Pending
------------------------------------------------------------------------------
Running 2/2 task(s) with status Pending

rng.build: executing ./build.sh build in "/home/fho/git/baur-example/project-pink-einstein/rng" failed: exit status 1
terminating, skipping execution of queued task runs
unixtime.build: execution successful (731.618ms)
unixtime.build: created file: dist/unixtime (size: 8.531 KiB)
task execution finished, waiting for uploads to finish...
unixtime.build: file: dist/unixtime uploaded to /tmp/baur_build_artifacts/unixtime-e4d7dc4a4eeca92809ab2f5f0f12668d66e13e36/unixtime (96.153 MiB/s)
unixtime.build: run stored in database with ID 1
------------------------------------------------------------------------------
finished in: 784.358ms

This is a good opportunity to add a cmdline parameter to disable fail fast.