When dynamic config is mis-specified say what key is incorrect (#371)

I improved the error message that appears when you specify dynamic config incorrectly; the previous message  `Error: dynamic config value not in KEY=JSON_VAL format` isn't helpful when you're specifying more than a few values

server/commands.go

@@ -362,7 +362,7 @@ func getDynamicConfigValues(input []string) (map[dynamicconfig.Key][]dynamicconf
 	for _, keyValStr := range input {
 		keyVal := strings.SplitN(keyValStr, "=", 2)
 		if len(keyVal) != 2 {
-			return nil, fmt.Errorf("dynamic config value not in KEY=JSON_VAL format")
+			return nil, fmt.Errorf("dynamic config value %s not in KEY=JSON_VAL format", keyValStr)
 		}
 		key := dynamicconfig.Key(keyVal[0])
 		// We don't support constraints currently