Hmm, I just looked at it again, and I am kind of arriving at the same logic again (which was there before I made changes, I really only refactored it into two routines):
Looking at `setup.py` [where the package checks are called](https://github.com/matplotlib/matplotlib/blob/v1.3.x/setup.py#L144-L150) any raised `CheckFailed` Exception drops out of the installation (ignoring any install message assigned in there like "installing") and its Exception message is [used exclusively to indicate the reason](https://github.com/matplotlib/matplotlib/blob/v1.3.x/setup.py#L149-L152). Only if `check_requirements()` properly returns (no `raise`), this string is appended and the package is assigned for installing in `setup.py`.

Summary..
 - configuration opt out -> `check()` raises -> "no [skipping due to configuration]"
 - dependency check fails -> `check_requirements()` raises -> "no [exception msg as defined in overridden `check_requirements()`]" 
 - normal install -> `check_requirements()` returns string -> "yes ['installing' + ', ' + custom message by `check_requirements()`]"
 - install forced ->  `check_requirements()` returns string -> "yes ['installing forced (config override)' + ', ' + custom message by `check_requirements()`]"

Let me know if I am missing something here..