urlapi: add curl_url_strerror()

Add curl_url_strerror() to convert CURLUcode into readable string and
facilitate easier troubleshooting in programs using URL API.
Extend CURLUcode with CURLU_LAST for iteration in unit tests.
Update man pages with a mention of new function.
Update example code and tests with new functionality where it fits.

Closes #7605

lib/strerror.c

@@ -453,6 +453,78 @@ curl_share_strerror(CURLSHcode error)
 #endif
 }
 
+const char *
+curl_url_strerror(CURLUcode error)
+{
+#ifndef CURL_DISABLE_VERBOSE_STRINGS
+  switch(error) {
+  case CURLUE_OK:
+    return "No error";
+
+  case CURLUE_BAD_HANDLE:
+    return "An invalid CURLU pointer was passed as argument";
+
+  case CURLUE_BAD_PARTPOINTER:
+    return "An invalid 'part' argument was passed as argument";
+
+  case CURLUE_MALFORMED_INPUT:
+    return "A malformed input was passed to a URL API function";
+
+  case CURLUE_BAD_PORT_NUMBER:
+    return "The port number was not a decimal number between 0 and 65535";
+
+  case CURLUE_UNSUPPORTED_SCHEME:
+    return "This libcurl build doesn't support the given URL scheme";
+
+  case CURLUE_URLDECODE:
+    return "URL decode error, most likely because of rubbish in the input";
+
+  case CURLUE_OUT_OF_MEMORY:
+    return "A memory function failed";
+
+  case CURLUE_USER_NOT_ALLOWED:
+    return "Credentials was passed in the URL when prohibited";
+
+  case CURLUE_UNKNOWN_PART:
+    return "An unknown part ID was passed to a URL API function";
+
+  case CURLUE_NO_SCHEME:
+    return "There is no scheme part in the URL";
+
+  case CURLUE_NO_USER:
+    return "There is no user part in the URL";
+
+  case CURLUE_NO_PASSWORD:
+    return "There is no password part in the URL";
+
+  case CURLUE_NO_OPTIONS:
+    return "There is no options part in the URL";
+
+  case CURLUE_NO_HOST:
+    return "There is no host part in the URL";
+
+  case CURLUE_NO_PORT:
+    return "There is no port part in the URL";
+
+  case CURLUE_NO_QUERY:
+    return "There is no query part in the URL";
+
+  case CURLUE_NO_FRAGMENT:
+    return "There is no fragment part in the URL";
+
+  case CURLUE_LAST:
+    break;
+  }
+
+  return "CURLUcode unknown";
+#else
+  if(error == CURLUE_OK)
+    return "No error";
+  else
+    return "Error";
+#endif
+}
+
 #ifdef USE_WINSOCK
 /* This is a helper function for Curl_strerror that converts Winsock error
  * codes (WSAGetLastError) to error messages.