Document any crashes from the user's text editor

Rather than failing with no information, display the child exit
status and the command line we tried to use to edit a text file.
There may be some useful information to help understand the crash.

Signed-off-by: Shawn O. Pearce <sop@google.com>

error.py

@@ -24,6 +24,11 @@ class ManifestInvalidRevisionError(Exception):
 class EditorError(Exception):
   """Unspecified error from the user's text editor.
   """
+  def __init__(self, reason):
+    self.reason = reason
+
+  def __str__(self):
+    return self.reason
 
 class GitError(Exception):
   """Unspecified internal error from git.