Return a generic "Translation failed" message instead of echoing
err.Error() in the HTTP response, and log the underlying error
server-side via log.Printf. Avoids leaking internal details (proxy
address, upstream URL, etc.) to API clients while keeping the 500
behavior introduced in #224.
The two handlers ignored the error from c.BindJSON, so a malformed JSON body was silently processed as an empty payload. Check the error and return 400, consistent with /v2/translate.
The /translate, /v1/translate and /v2/translate handlers called log.Fatalf when TranslateByDeepLX returned an error. log.Fatalf calls os.Exit, so a single failed translation would tear down the whole server process. Return a 500 JSON response and keep serving instead.
