Improve error message when --mpi used incorrectly

[joezuntz]

Some stages have parallel=False set and so cannot be run under MPI. The current error message just says that there is no flag --mpi for the stage; this makes it a clearer message.

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (9027b67) compared to base (497a005). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #86 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 13 13 Lines 1527 1532 +5 ========================================= + Hits 1527 1532 +5 ``` | [Impacted Files](https://codecov.io/gh/LSSTDESC/ceci/pull/86?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC) | Coverage Δ | | |---|---|---| | [ceci/stage.py](https://codecov.io/gh/LSSTDESC/ceci/pull/86/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#diff-Y2VjaS9zdGFnZS5weQ==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC)

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[joezuntz]

I've also moved some of the tests that were previously separate into subprocesses of the main command so that it an properly track the code coverage. This seems to work!