histedit: abort gracefully on --continue/--abort with no state

Previously we'd print an ugly message saying that the histedit-state file
doesn't exist in the repo.

hgext/histedit.py

@@ -147,6 +147,7 b' try:'
     pickle.dump # import now
 except ImportError:
     import pickle
+import errno
 import os
 import sys
 
@@ -741,7 +742,12 b' def writestate(repo, parentnode, rules, '
 def readstate(repo):
     """Returns a tuple of (parentnode, rules, keep, topmost, replacements).
     """
-    fp = open(os.path.join(repo.path, 'histedit-state'))
+    try:
+        fp = open(os.path.join(repo.path, 'histedit-state'))
+    except IOError, err:
+        if err.errno != errno.ENOENT:
+            raise
+        raise util.Abort(_('no histedit in progress'))
     return pickle.load(fp)
 
 

tests/test-histedit-arguments.t

@@ -41,6 +41,16 b' Repo setup.'
        one
   
 
+histedit --continue/--abort with no existing state
+--------------------------------------------------
+
+  $ hg histedit --continue
+  abort: no histedit in progress
+  [255]
+  $ hg histedit --abort
+  abort: no histedit in progress
+  [255]
+
 Run a dummy edit to make sure we get tip^^ correctly via revsingle.
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