transactions: don't show a backtrace when journal exists

Now we simply print a message on how to recover.

tests/test-journal-exists

@@ -0,0 +1,12 b''
+#!/bin/bash
+
+hg init
+echo a > a
+hg ci -Am0
+
+touch .hg/store/journal
+
+echo foo > a
+hg ci -Am0
+
+hg recover

tests/test-journal-exists.out

@@ -0,0 +1,8 b''
+adding a
+abort: journal already exists - run hg recover!
+rolling back interrupted transaction
+checking changesets
+checking manifests
+crosschecking files in changesets and manifests
+checking files
+1 files, 1 changesets, 1 total revisions

mercurial/localrepo.py

@@ -519,6 +519,10 b' class localrepository(repo.repository):'
         if self._transref and self._transref():
             return self._transref().nest()
 
+        # abort here if the journal already exists
+        if os.path.exists(self.sjoin("journal")):
+            raise repo.RepoError(_("journal already exists - run hg recover"))
+
         # save dirstate for rollback
         try:
             ds = self.opener("dirstate").read()

mercurial/transaction.py

@@ -18,10 +18,6 b' class transaction(object):'
     def __init__(self, report, opener, journal, after=None):
         self.journal = None
 
-        # abort here if the journal already exists
-        if os.path.exists(journal):
-            raise AssertionError(_("journal already exists - run hg recover"))
-
         self.count = 1
         self.report = report
         self.opener = opener