60 handle errors from non http url sources

[EddyCMWF]

ftp is not compatible with multiurl, so if ftp we use the wget downloader.
Hopefully the approach is generic to facilitate other downloaders in the future.

[EddyCMWF]

Hi Mattia,
I've fixed and released multiurl which addresses the issue upstream of cads-adaptors.
I've also added an ftp download test.
So good for a re-review.
Eddy

[malmans2]

I'll take a look tomorrow (bank holiday in Italy today).
I you didn't notice it, the other day I opened a PR to this branch to test both ftp ad http: #65
It also fixes this #61 (comment)

[malmans2]

Hi @EddyCMWF,

I have a few comments. I'm approving so you can merge when you are happy with it.

About the tests: As I mentioned in the comments, I'm a bit worried that the library used in this PR is not stable/reliable.
Also, IMO it's better if we don't rely on external files. That way we don't need internet access to run the tests, we don't download external files every time tests are executed, and we are not affected by the external data (e.g., urls could change, downtimes, ...)
Given that the ftp fixture is just about 10 lines of code, I've aligned another branch to this PR in case you change your mind and you want to use my version of the tests: #66