much improved use of MoveFileTransacted and friends.

Turns out I was letting a lot of errors silently pass, meaning
transactional operations would appear to work but then silently fail
when calling commit().

Also refactored to allow use of the fallback implementation when
transactions are offered by the OS but not supported by the target
filesystem.

ChangeLog.txt

@@ -1,6 +1,10 @@
 
 v0.8.5:
 
+    * FSTransaction:  better error handling, and better detection of cases
+      where native transactions will fail (e.g. on a FAT32 USB device).
+    * FSTransaction:  accept an optional "root" parameter, and enforce all
+      operations to be inside this root.
     * bdist_esky:  accept wider variety of objects for bootstrap_code option
       and Executable.script attribute. Lists are concatenated, source is
       obtained using inspect.getsource().

esky/__init__.py

@@ -809,7 +809,7 @@ def install_version(self,version):
         try:
             if not os.path.exists(target):
                 os.rename(source,target)
-            trn = esky.fstransact.FSTransaction()
+            trn = esky.fstransact.FSTransaction(self.appdir)
             try:
                 self._unpack_bootstrap_env(version,trn)
             except Exception:
@@ -878,7 +878,7 @@ def uninstall_version(self,version):
             #  Clean up the bootstrapping environment in a transaction.
             #  This might fail on windows if the version is locked.
             try:
-                trn = esky.fstransact.FSTransaction()
+                trn = esky.fstransact.FSTransaction(self.appdir)
                 try:
                     self._cleanup_bootstrap_env(version,trn)
                 except Exception: