Detect when hpc report (--coverage) is giving a trivial "100%"

[mgsloan]

Sometimes when things don't go right with coverage, it yields a report with lots of (0/0):

Generating HTML coverage report for cereal-conduit's test-suite "test-cereal-conduit"
100% expressions used (0/0)
100% boolean coverage (0/0)
     100% guards (0/0)
     100% 'if' conditions (0/0)
     100% qualifiers (0/0)
100% alternatives used (0/0)
100% local declarations used (0/0)
100% top-level declarations used (0/0)

This happens due to the way cereal-conduit's cabal file is written (see #1008)

Skimming by, this might look like a stellar coverage report - all 100%s! However, it isn't really checking anything. (0/0) means that no top level definitions were included in the report. Unfortunately, the hpc program tends to carry on with yielding a report instead of throwing errors in various circumstances. We should let the user know that this circumstance might indicate a stack bug, and perhaps link them to this issue.

[mgsloan]

Note: since we aren't going to add a warning for #1008, the message for this should mention that it's a possible cause. I'm imagining a message like this:

Error: The coverage report for cereal-conduit's test-suite "test-cereal-conduit" did not consider any code. One possible cause of this is if your test-suite builds the library code (see stack issue #1008). It may also indicate a bug in stack or the hpc program. Please report this issue if you think your coverage report should have meaningful results.

[mgsloan]

Done!