Add error message to catch a plugin being added as a middleware

[rajeshsegu]

No description provided.

[KevinGrandon]

On the fence about this one as it seems that flow does catch this case. I'm not sure we want to try to implement runtime checks for everything that we do have flow types for, but let's see what people think.

[lhorie]

Need to fix flow error. Also, this is not greenkeeping. I'd say bugfix might be a more appropriate tag.

@KevinGrandon I think it's ok to add runtime checks for these, at least for now. We've had a few cases of post-migration code throwing bad errors and wasting people's time that could've been avoided by surfacing errors such as this. Also, the test suite already has a lot of tests similar to this one.

[KevinGrandon]

Ok, works for me then. Requesting changes to clear review queue. Let's get flow issues fixed and request re-reivew. Thanks!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@fb86e7f). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #349   +/-   ##
=========================================
  Coverage          ?   93.23%           
=========================================
  Files             ?       18           
  Lines             ?      458           
  Branches          ?       92           
=========================================
  Hits              ?      427           
  Misses            ?       17           
  Partials          ?       14

Impacted Files	Coverage Δ
src/base-app.js	90.11% <100%> (ø)

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[rajeshsegu]

@KevinGrandon @lhorie please do review ( updated tag to bugfix and flow passes )