Update ElectrumResponseException.php

If wallet does not enough money for transaction, response structure:
[
id => 0
error => message (string)
]

Author: kbgod

src/Response/Exception/ElectrumResponseException.php

@@ -23,10 +23,18 @@ public static function createFromElectrumResponse(array $response)
         $message = '';
         $code = 0;
 
-        if (isset($response['error']['message'])) {
+        if (isset($response['error'])) {
+            $text = '';
+            if(is_string($response['error'])) {
+                $text = $response['error'];
+            } elseif (is_array($response['error'])
+                and isset($response['error']['message'])
+                and is_string($response['error']['message'])) {
+                $text = $response['error']['message'];
+            }
             $message = vsprintf(
                 'Electrum API returned error: `%s`',
-                $response['error']['message']
+                $text
             );
         }
 
@@ -36,4 +44,4 @@ public static function createFromElectrumResponse(array $response)
 
         return new self($message, $code);
     }
-}
+}