Improve error message for bad url

[jcamiel]

Given the following (invalid) Hurl file:

GET "https://example.org"

We get this error message:

$ echo 'GET "https://example.org"' | hurl
error: Error { pos: Pos { line: 1, column: 5 }, recoverable: false, inner: Url }
  --> -:1:5
   |
 1 | GET "https://example.org"
   |     ^ Error { pos: Pos { line: 1, column: 5 }, recoverable: false, inner: Url }
   |

Maybe we can improve the message for this "common" case (and not leak Error inner structure).