Fix set -e for conditionals

When porting scripts to Ion I noticed that using set -e scripts would exit early even when conditions fail. This is in contrast to common shells like Bash which has a list of exceptions exactly for this case.

Approach

In this PR I've tried to tackle the problem in the most naive way:

• keep creating PipelineError::EarlyExit to "capture intent"
• prevent result/error propagation for PipelineError::EarlyExit in certain locations
• re-raise if necessary

I've also added specific test cases to test proper behavior and copied tests for conditionals to also test with set -e.

Known Problems

• I've currently no idea how to ignore failing subshells in conditions ( see if ! $(false | true) ... below)

Please feel free to rewrite my PR or find a better approach to this issue.