Improve Error Message "Could not Resolve Host"

[fabricereix]

Display url/hostname used when host can not be resolved.
It is useful when url is calculated from a variable.

For example, currently hurl produces the following error message.

$ echo 'GET {{url}}' | hurl --variable url=http://unknown 
error: Http Connection
  --> -:1:5
   |
 1 | GET {{url}}
   |     ^^^^^^^ Could not resolve host
   |

The following one will be be more helpful

$ echo 'GET {{url}}' | hurl --variable url=http://unknown 
error: Http Connection
  --> -:1:5
   |
 1 | GET {{url}}
   |     ^^^^^^^ Could not resolve host <unknown>
   |