Ignore shell jobs return values when being killed

Current behavior

• Shell jobs may write return values to a file provided via the JS7_RETURN_VALUES environment variable.
• When the job is killed (maybe due to timeout) while the file is being written, the return values may be incomplete or inconsistent.

Desired behavior

• The return values of a killed shell job must be ignored.
• Only $returnCode is returned.
• Internal Java jobs are not affected because they are in control of results.