fix(unit): don't panic on bad lines in unit files
fix(unit): propagate NewUnit errors
chore(agent): centralize NewJob/NewUnit invocations in agent test
chore(fleetctl): centralize NewJob/NewUnit invocation in fleetctl tests
chore(agent): centralize NewJob/NewUnit invocations in agent state test
chore(job): centralize NewUnit invocations in job_test
chore(registry): handle NewUnit errors in registry tests
chore(sign): handle NewUnit errors in signing tests
Merge pull request #546 from jonboulle/546

fix(unit): fix panic on malformed unit files, and clean up tests