errdefs: FromStatusCode() don't log "FIXME" debug message

This utility is used by the client, which cannot do anything about errors
received from the API. In situations where no API connection was possible,
for example, if the client has no permissions to connect to the socket,
the request would have a "-1" status-code;
https://github.com/moby/moby/blob/3e39ec60dab859f70577ea8561fe5bda5236749e/client/request.go#L133-L134

In this case, a client with "debug" enabled, would print _and_ log a confusing
error message:

    DEBU[0000] FIXME: Got an status-code for which error does not match any expected type!!!  error="Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Post \"http://%2Fvar%2Frun%2Fdocker.sock/v1.24/build?buildargs=%7B%7D&cachefrom=%5B%5D&cgroupparent=&cpuperiod=0&cpuquota=0&cpusetcpus=&cpusetmems=&cpushares=0&dockerfile=Dockerfile.repro&labels=%7B%7D&memory=0&memswap=0&networkmode=default&rm=1&shmsize=0&t=repro&target=&ulimits=null&version=1\": dial unix /var/run/docker.sock: connect: permission denied" module=api status_code=-1
    Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Post "http://%2Fvar%2Frun%2Fdocker.sock/v1.24/build?buildargs=%7B%7D&cachefrom=%5B%5D&cgroupparent=&cpuperiod=0&cpuquota=0&cpusetcpus=&cpusetmems=&cpushares=0&dockerfile=Dockerfile.repro&labels=%7B%7D&memory=0&memswap=0&networkmode=default&rm=1&shmsize=0&t=repro&target=&ulimits=null&version=1": dial unix /var/run/docker.sock: connect: permission denied

In the above; `DEBU` logs the error (including the "FIXME"), and the second
line is the error message printed.

This was a mistake on my side when I added the `FromStatusCode` utility. I
implemented that to be the counterpart to `FromError`, but in doing so also
copied over the logging (see 1af30c50ca0ad81e2839ff4f2c5e70413f021d52). That
log-message is only intended to be logged on the daemon side, for situations
where we return an error without a proper errdefs (which would result in an
500 "internal server error" to be returned by the API).

This patch removes the debug log, and a minor cleanup to explicitly return
"nil" if we didn't get an error in the first place.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

errdefs/http_helpers.go

@@ -2,14 +2,12 @@ package errdefs // import "github.com/docker/docker/errdefs"
 
 import (
 	"net/http"
-
-	"github.com/sirupsen/logrus"
 )
 
 // FromStatusCode creates an errdef error, based on the provided HTTP status-code
 func FromStatusCode(err error, statusCode int) error {
 	if err == nil {
-		return err
+		return nil
 	}
 	switch statusCode {
 	case http.StatusNotFound:
@@ -33,11 +31,6 @@ func FromStatusCode(err error, statusCode int) error {
 			err = System(err)
 		}
 	default:
-		logrus.WithError(err).WithFields(logrus.Fields{
-			"module":      "api",
-			"status_code": statusCode,
-		}).Debug("FIXME: Got an status-code for which error does not match any expected type!!!")
-
 		switch {
 		case statusCode >= 200 && statusCode < 400:
 			// it's a client error