To be more clear, I have previously been able to run the ./install.sh script from forge - before forge was changed to download mcp by it self.
This was due to the fact that mcp actually includes the astyle-osx in it's runtime/bin directory.
So it is not a matter of detecting it nor throwing an error - but just make sure it has the execute flag, the mcp zip does have this flag configured but somehow forge doesn't respect this when unpacking it, so my guess without having looking into the code that handles the unpacking is that the problem is a missing parameter or similar on the call to unzip.