Fix handling of squeue call with invalid jobid

[giffels]

In case a job is already completed and only one non-existing job id is provided the squeue command is failing with exit code 1 and it prints "Invalid job id specified" to stderr. If two or more non-existing job ids or a mixture of existing ones and non-existings jobs are provided, squeue command succeeds.

This pull request is adjusting the handling of failed squeue commands correspondigly.

Thanks to @rodwalker for reporting it.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (10072ef) 98.87% compared to head (d105f23) 98.87%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #326 +/- ## ======================================= Coverage 98.87% 98.87% ======================================= Files 55 55 Lines 2222 2225 +3 ======================================= + Hits 2197 2200 +3 Misses 25 25 ```

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