Optionally continue with installation even if some package fails

[PaulDance]

Currently, whenever the installation of some package fails, the whole process fails on the first error encountered. That may not be ideal in some situations. Therefore, adding a ship --keep-going CLI option to allow ignoring installation errors only would be useful.

Both crate::cargo::install and install_all should still return a Result and not only an Ok:

• install should not be changed.
• install_all should accept a boolean argument enabling this behavior:
  • when false, nothing should be different from now;
  • when true, errors should be collected into a composite error value, potentially a simple vector, and returned as an Err so it would still be reported.

[PaulDance]

Actually, the failure needs to be implemented first :grimacing: