formula_installer: handle post_install exceptions.

Warn users and point them to `brew postinstall` (which can provide a
backtrace).

Closes Homebrew/homebrew#21887.

Library/Homebrew/formula_installer.rb

@@ -118,7 +118,11 @@ class FormulaInstaller
       clean
     end
 
-    f.post_install
+    begin
+      f.post_install
+    rescue
+      opoo "#{f.name} post_install failed. Rerun with `brew postinstall #{f.name}`."
+    end
 
     opoo "Nothing was installed to #{f.prefix}" unless f.installed?
   end