[PLUGIN-1756] Fixed URL Validation

Fixed URL Validation

When an invalid url is given, the validation logic only checks for a valid URI which may not raise errors for a lot of strings that are not valid URLs. Replacing the validation to check for a URL will fix the proper error.
Some exceptions are thrown by the http library without any error message string. This is usually present on the cause of the error.

Adding this code snippet will fix null issue for other exceptions thrown by the library.

String errorMessage = e.getMessage();
if (errorMessage == null && e.getCause() != null) {
errorMessage = e.getCause().getMessage();
}