Solver cabal install hint

[adrianwong]

Resolves #3841.

Recover from all errors on invoking proc "cabal" --numeric-version.

This reintroduces the helpful hint to stack install cabal-install if stack solver is run when cabal is not on PATH, instead of displaying the exception message propagated from RIO (thanks for the help, @mgsloan!).

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

• Ran stack solver on a project using an existing version of stack, with and without cabal on PATH
• Ran stack solver on the same project using the modified stack, with and without cabal on PATH. Verified that the helpful hint was displayed without cabal on PATH; verified that stack solver ran without issue with cabal on PATH

[adrianwong]

Note: closed and resubmitted in #4100 to rerun tests, as Travis was being flaky. Should have just reopened.