statichttprepo: explicitly convert error message to str (issue6247)

For Python 2.7, the implicit conversion of the HTTPError instance to
str was good enough. For Python 3.x, this fails later when hitting the
str to bytes conversion logic.

Differential Revision: https://phab.mercurial-scm.org/D9661

mercurial/statichttprepo.py

@@ -61,7 +61,10 b' class httprangereader(object):'
             code = f.code
         except urlerr.httperror as inst:
             num = inst.code == 404 and errno.ENOENT or None
-            raise IOError(num, inst)
+            # Explicitly convert the exception to str as Py3 will try
+            # convert it to local encoding and with as the HTTPResponse
+            # instance doesn't support encode.
+            raise IOError(num, str(inst))
         except urlerr.urlerror as inst:
             raise IOError(None, inst.reason)