error: add CensoredNodeError, will be thrown when content deliberately erased

This change introduces the error plus a corresponding catch in dispatch, to
provide localized error messages.

The verb "censor" is used in this commit and all following to refer to erasing
the content of a revlog revision (filelog, for now) without recalculating node
IDs, leaving that revision invalid. Further work must be done to safely share
such revision data with compliant clients.

I find the analogy to censorship straightforward; for less politically
charged options, consider "erase", "excise", "expunge", or "blackhole".

mercurial/dispatch.py

@@ -193,6 +193,8 b' def _runcatch(req):'
             ui.warn(_(" empty string\n"))
         else:
             ui.warn("\n%r\n" % util.ellipsis(inst.args[1]))
+    except error.CensoredNodeError, inst:
+        ui.warn(_("abort: file censored %s!\n") % inst)
     except error.RevlogError, inst:
         ui.warn(_("abort: %s!\n") % inst)
     except error.SignalInterrupt:

mercurial/error.py

@@ -117,3 +117,9 b' class ReadOnlyPartError(RuntimeError):'
     """error raised when code tries to alter a part being generated"""
     pass
 
+class CensoredNodeError(RevlogError):
+    """error raised when content verification fails on a censored node"""
+
+    def __init__(self, filename, node):
+        from node import short
+        RevlogError.__init__(self, '%s:%s' % (filename, short(node)))