Don't write weights file if old file fails to load

Issue #13

readme.md

@@ -172,3 +172,7 @@ type "ax[3" or "ax 3" (ax-space-space-3) it will only match "Axis
 * `v1.8` (wip)
 
   * Installation instructions updated to support Nuke 9
+
+  * Weights file no longer overwritten if it fails to load for some
+    reason.
+    [Github issue #13](https://github.com/dbr/tabtabtab-nuke/issues/13)

tabtabtab.py

@@ -148,6 +148,7 @@ class NodeWeights(object):
     def __init__(self, fname = None):
         self.fname = fname
         self._weights = {}
+        self._successful_load = False
 
     def load(self):
         if self.fname is None:
@@ -165,16 +166,24 @@ def _load_internal():
         # Catch any errors, print traceback and continue
         try:
             _load_internal()
+            self._successful_load = True
         except Exception:
             print "Error loading node weights"
             import traceback
             traceback.print_exc()
+            self._successful_load = False
 
     def save(self):
         if self.fname is None:
             print "Not saving node weights, no file specified"
             return
 
+        if self._successful_load:
+            # Avoid clobbering existing weights file on load error
+            print "Not writing weights file because %r previously failed to load" % (
+                self.fname)
+            return
+
         def _save_internal():
             import json
             ndir = os.path.dirname(self.fname)