fix(log): Handle case when error code is missing (#303)

# Before
```
HPM] Error occurred while trying to proxy request <redacted> from <redacted> to <redacted> (undefined) (https://nodejs.org/api/errors.html#errors_common_system_errors)
```

# After
```
[HPM] Error occurred while trying to proxy request <redacted> from <redacted> to <redacted> (Error: Hostname/IP doesn't match certificate's altnames: "Host: <redacted>. is not in the cert's altnames: DNS:<redacted>, DNS:<redacted>") (https://nodejs.org/api/errors.html#errors_common_system_errors)
```

Author: sorin-davidoi

lib/index.js

@@ -147,6 +147,6 @@ function HttpProxyMiddleware (context, opts) {
     var errorMessage = '[HPM] Error occurred while trying to proxy request %s from %s to %s (%s) (%s)'
     var errReference = 'https://nodejs.org/api/errors.html#errors_common_system_errors' // link to Node Common Systems Errors page
 
-    logger.error(errorMessage, req.url, hostname, target, err.code, errReference)
+    logger.error(errorMessage, req.url, hostname, target, err.code || err, errReference)
   }
 }