Replace Unicode quotes in user-facing strings by ASCII

Relevant RFC: NixOS/rfcs#4

$ ag -l | xargs sed -i -e "/\"/s/’/'/g;/\"/s/‘/'/g"

src/nix/log.cc

@@ -39,7 +39,7 @@ struct CmdLog : InstallablesCommand
                     break;
                 }
                 if (!found)
-                    throw Error("build log of path ‘%s’ is not available", path);
+                    throw Error("build log of path '%s' is not available", path);
             }
         }
     }