mq: clarify the fact that qimport is trying to read a file

"hg qimport tip" would throw "abort: unable to read tip" before this.

hgext/mq.py

@@ -1702,7 +1702,7 b' class queue(object):'
                     else:
                         text = url.open(self.ui, filename).read()
                 except (OSError, IOError):
-                    raise util.Abort(_("unable to read %s") % filename)
+                    raise util.Abort(_("unable to read file %s") % filename)
                 if not patchname:
                     patchname = normname(os.path.basename(filename))
                 self.check_reserved_name(patchname)

tests/test-mq-qimport-fail-cleanup.out

@@ -4,7 +4,7 b' adding a'
 
 #qimport valid patch followed by invalid patch
 adding b.patch to series file
-abort: unable to read fakepatch
+abort: unable to read file fakepatch
 
 #valid patches before fail added to series
 b.patch

tests/test-mq-qimport.out

@@ -1,5 +1,5 b''
 % qimport non-existing-file
-abort: unable to read non-existing-file
+abort: unable to read file non-existing-file
 % import email
 adding email to series file
 applying email