Fix do_install for missing keys, rearrange lines in conf

install.py

@@ -119,14 +119,25 @@ class Installer:
 
         if not packages:
             try:
-                packages = tuple(
-                    os_specific_packages[self.operating_system]
-                    + os_specific_packages["both"]
-                )
+                # dict only contains os-specific packages
+                if "both" not in os_specific_packages.keys():
+                    packages = tuple(
+                        os_specific_packages[self.operating_system]
+                    )
+                # some packages for other operating systems are in the dict
+                elif self.operating_system not in os_specific_packages.keys():
+                    packages = tuple(
+                        os_specific_packages["both"]
+                    )
+                # dict contains specific and non-specific packages
+                else:
+                    packages = tuple(
+                        os_specific_packages[self.operating_system]
+                        + os_specific_packages["both"]
+                    )
 
             except KeyError:
-                ui.fatal("Operating System not understood.")
-                return
+                ui.warning("No packages for {}".format(self.operating_system))
 
         failed_installs = []