Undefined error message for various --output error

[jcamiel]

On Hurl 4.1.0:

Output file doesn't exist:

$ echo 'GET https://hurl.dev' | hurl --output /tmp/toto/tata.out
error: Undefined error

Output file is readonly:

$ echo 'GET https://hurl.dev' | hurl --output /tmp/readonly.out
error: Undefined error

Output file is a directory:

$ echo 'GET https://hurl.dev' | hurl --output /tmp/dir/
error: Undefined error

Expected: error message should be informative