transaction: display data about why the transaction failed to rollback

We saw more of these a while back. Having more data available would be nice.

mercurial/transaction.py

@@ -21,6 +21,9 b' from . import ('
     pycompat,
     util,
 )
+from .utils import (
+    stringutil,
+)
 
 version = 2
 
@@ -582,8 +585,10 b' class transaction(util.transactional):'
                           self._vfsmap, self._entries, self._backupentries,
                           False, checkambigfiles=self._checkambigfiles)
                 self._report(_("rollback completed\n"))
-            except BaseException:
+            except BaseException as exc:
                 self._report(_("rollback failed - please run hg recover\n"))
+                self._report(_("(failure reason: %s)\n")
+                             % stringutil.forcebytestr(exc))
         finally:
             self._journal = None
             self._releasefn(self, False) # notify failure of transaction

tests/test-repair-strip.t

@@ -51,6 +51,7 b''
   transaction abort!
   failed to truncate data/b.i
   rollback failed - please run hg recover
+  (failure reason: [Errno 13] Permission denied .hg/store/data/b.i')
   strip failed, backup bundle
   abort: Permission denied .hg/store/data/b.i
   % after update 0, strip 2
@@ -104,6 +105,7 b''
   transaction abort!
   failed to truncate 00manifest.i
   rollback failed - please run hg recover
+  (failure reason: [Errno 13] Permission denied .hg/store/00manifest.i')
   strip failed, backup bundle
   abort: Permission denied .hg/store/00manifest.i
   % after update 0, strip 2