Check whether we can write to our install directory before going any further

and report a useful error message in the event that we cannot.

src/java/com/threerings/getdown/launcher/Getdown.java

@@ -88,6 +88,14 @@ public class Getdown extends Thread
             return;
         }
 
+        // determine whether or not we can write to our install directory
+        File instdir = _app.getLocalPath("");
+        if (!instdir.canWrite()) {
+            updateStatus(MessageUtil.tcompose("m.readonly_error", instdir));
+            _dead = true;
+            return;
+        }
+
         try {
             _dead = false;
             if (detectProxy()) {

src/java/com/threerings/getdown/messages.properties

@@ -60,6 +60,10 @@ m.init_error = The application has failed to launch due to the following \
   error:\n{0}\n\nPlease visit\n{1} for \
   information on how to handle such problems.
 
+m.readonly_error = The directory in which this application is installed:\n \n\
+  {0}\n \nis read-only. Please install the applicaton into a directory where \
+  you have write access.
+
 m.missing_resource = The application has failed to launch due to a \
   missing resource:\n{0}\n\nPlease visit\n{1} for information on how to handle such problems.