Revert "run macports check after failed build instead of before every build"

CannotInstallFormulaError is only raised when another version of the
formula is linked or any of its dependencies are unlinked, in which case
this warning is totally irrelevant. It should be checked after build
errors instead.

This reverts commit 466d0f718bc2241e830393b384294c21194995e0.
Check for MacPorts/Fink installation only after build failures