Enable golangci-lint (#49)

## Summary:
It would be nice to have some linting beyond `go vet`!  Now we do.  I
started by copying the config from Khan/webapp.  I did remove a couple
of staticcheck checks that I didn't feel were useful.  (Note also that
exportloopref is the replacement for scopelint in newer golangci-lint.)

Included are all the needed lint fixes; most are stylistic but the
changes in the example are a (minor) bugfix.

Fixes #22.
Issue: https://github.com/Khan/genqlient/issues/22

## Test plan:
make check

Author: benjaminjkraft

Reviewers: aberkan, dnerdy, benjaminjkraft, csilvers, MiguelCastillo

Required Reviewers: 

Approved by: aberkan, dnerdy

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint

Pull request URL: https://github.com/Khan/genqlient/pull/49

graphql/client.go

@@ -46,9 +46,9 @@ type Client interface {
 }
 
 type client struct {
+	httpClient *http.Client
 	endpoint   string
 	method     string
-	httpClient *http.Client
 }
 
 // NewClient returns a Client which makes requests to the given endpoint,
@@ -64,7 +64,7 @@ func NewClient(endpoint string, httpClient *http.Client) Client {
 	if httpClient == nil {
 		httpClient = http.DefaultClient
 	}
-	return &client{endpoint, http.MethodPost, httpClient}
+	return &client{httpClient, endpoint, http.MethodPost}
 }
 
 type payload struct {
@@ -109,9 +109,10 @@ func (c *client) MakeRequest(ctx context.Context, opName string, query string, r
 	defer resp.Body.Close()
 
 	if resp.StatusCode != http.StatusOK {
-		respBody, err := ioutil.ReadAll(resp.Body)
+		var respBody []byte
+		respBody, err = ioutil.ReadAll(resp.Body)
 		if err != nil {
-			respBody = []byte("<unreadable>")
+			respBody = []byte(fmt.Sprintf("<unreadable: %v>", err))
 		}
 		return fmt.Errorf("returned error %v: %s", resp.Status, respBody)
 	}