[smt] treat stop with non-zero ret like an assertion

[ekiwi]

We treat it as an assertion that the stop will never be enabled. stop(0) will still be ignored (but now demoted to a info from a warning).

Contributor Checklist

• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?

Type of Improvement

• smt backend code generation

API Impact

• demotes some warnings to info
• stop with non-zero return code are now emitted similar to assertions instead of being ignored

Backend Code Generation Impact

• only impacts smt/btor generation, no impact on Verilog

Desired Merge Strategy

• squash

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[jackkoenig]

LGTM and makes sense!