getting a meaningful error out of oauth token exchange

I've successfully used this goth library, however, I found that
golang/x/oauth2 eats the server's useful error during Exchange() returning:

oauth2: server response missing access_token

... instead of, for example:

invalid_team_for_non_distributed_app

I used this patch to oauth2 to get the error returned:

diff --git a/vendor/golang.org/x/oauth2/internal/token.go b/vendor/golang.org/x/oauth2/internal/token.go
index 0aa16905..3288de96 100644
--- a/vendor/golang.org/x/oauth2/internal/token.go
+++ b/vendor/golang.org/x/oauth2/internal/token.go
@@ -64,6 +64,7 @@ type tokenJSON struct {
        TokenType    string         `json:"token_type"`
        RefreshToken string         `json:"refresh_token"`
        ExpiresIn    expirationTime `json:"expires_in"` // at least PayPal returns string, while most return number
+       Error        string         `json:"error"`
 }
 
 func (e *tokenJSON) expiry() (t time.Time) {
@@ -216,7 +217,11 @@ func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string,
                // So just try both ways.
                authStyle = AuthStyleInParams // the second way we'll try
                req, _ = newTokenRequest(tokenURL, clientID, clientSecret, v, authStyle)
-               token, err = doTokenRoundTrip(ctx, req)
+               var err2 error
+               token, err2 = doTokenRoundTrip(ctx, req)
+               if err2 == nil {
+                       err = nil
+               }
        }
        if needsAuthStyleProbe && err == nil {
                setAuthStyle(tokenURL, authStyle)
@@ -245,8 +250,8 @@ func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
                        Body:     body,
                }
        }
+       var errorStr string
        var token *Token
        content, _, _ := mime.ParseMediaType(r.Header.Get("Content-Type"))
        switch content {
@@ -266,6 +271,7 @@ func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
                if expires != 0 {
                        token.Expiry = time.Now().Add(time.Duration(expires) * time.Second)
                }
+               errorStr = vals.Get("error")
        default:
                var tj tokenJSON
                if err = json.Unmarshal(body, &tj); err != nil {
@@ -279,9 +285,14 @@ func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
                        Raw:          make(map[string]interface{}),
                }
                json.Unmarshal(body, &token.Raw) // no error checks for optional fields
+               errorStr = tj.Error
        }
        if token.AccessToken == "" {
-               return nil, errors.New("oauth2: server response missing access_token")
+               err = errors.New("oauth2: server response missing access_token")
+               if errorStr != "" {
+                       err = errors.New(errorStr)
+               }
+               return nil, err
        }
        return token, nil
 }

Does anyone have a better solution? My challenge now is were and how to maintain this patch.

Start by reviewing vendor/golang.org/x/oauth2/internal/token.go, especially RetrieveToken and doTokenRoundTrip, and trace how goth reaches Exchange(). Compare the vendored behavior with the upstream oauth2 package and determine how server error responses should be exposed. Done means a maintainable approach preserves meaningful token-exchange errors without relying on an unmaintained local patch.