client: don't swallow ResponseError inside open() _readhello() can generate meaningful ResponseError exceptions. However, open()'s exception handler swallows these, converting them into generic ServerErrors. Allow the original ResponseErrors to pass through.

from tests import common
import hglib
from hglib.util import b

class test_encoding(common.basetest):
    def test_basic(self):
        self.client = hglib.open(encoding='utf-8')
        self.assertEquals(self.client.encoding, b('utf-8'))