pollEndpoint: improve error output

[cryptix]

After seeing the comments on #933, I realized the output of the errors was not good.

It now prints the errors as strings. i.e.:

error="Get http://127.0.0.1:10044/version: dial tcp 127.0.0.1:10044: connection refused"

instead of

error="wrong url or endpoint error from http.Get() &url.Error{Op:\"Get\", RL:\"http://127.0.0.1:5836/version\", Err:(*net.OpError)(0xc208032600)}"

[cryptix]

re-opened to get the tests running again but it only affected the travis-ci/pr not /push.

The faillure was a known fluke:

--- FAIL: TestBootstrap (1.23 seconds)
    dht_test.go:314: connecting 30 dhts in a ring
    dht_test.go:77: dial attempt failed
01:38:44.557 CRITI   boguskey: TestBogusPrivateKey.Sign -- this better be a test! <autogenerated>:21
FAIL
FAIL    github.com/jbenet/go-ipfs/routing/dht   5.049s

ps: rebased