Report all missing packages from channel

[jaimergp]

Description

conda.exceptions has many classes to play with if packages are not found:

• When the user asks for a package to be installed but this is not available in the chosen channels, conda raises PackagesNotFoundError. For the message to mention the offending channels, this exception class needs a channels kwarg. Otherwise, it'll default to "missing from target environment" messaging. We were missing this kwarg, so the messaging was wrong.
• Additionally, it has PackageNotInstalledError, which takes a prefix kwarg too, and mentions it in the messaging. However, this only accepts a single package name 🤷

Fun fact: conda classic uses both, depending on the context, and there's no good reason imo. I'll submit an issue about this in conda/conda

This PR simply fixes our missing kwarg so the messaging is correct.

Before this PR:

$ conda create -n unused --dry-run pkg-does-not-exist --solver=libmamba
pkgs/r/noarch                                                 No change
pkgs/main/linux-64                                            No change
pkgs/main/noarch                                              No change
pkgs/r/linux-64                                               No change
Collect all metadata (repodata.json): done
Solving environment: failed
pkgs/main/linux-64                                          Using cache
pkgs/main/noarch                                            Using cache
pkgs/r/linux-64                                             Using cache
pkgs/r/noarch                                               Using cache
Collect all metadata (repodata.json): done
Solving environment: failed

PackagesNotFoundError: The following packages are missing from the target environment:
  - pkg-does-not-exist

After this PR:

$ conda create -n unused --dry-run pkg-does-not-exist --solver=libmamba

pkgs/main/linux-64                                            No change
pkgs/main/noarch                                              No change
pkgs/r/linux-64                                               No change
pkgs/r/noarch                                                 No change
Collect all metadata (repodata.json): done
Solving environment: failed
pkgs/main/linux-64                                          Using cache
pkgs/main/noarch                                            Using cache
pkgs/r/linux-64                                             Using cache
pkgs/r/noarch                                               Using cache
Collect all metadata (repodata.json): done
Solving environment: failed

PackagesNotFoundError: The following packages are not available from current channels:

  - pkg-does-not-exist

Current channels:

  - defaults

To search for alternate channels that may provide the conda package you're
looking for, navigate to

    https://anaconda.org

and use the search bar at the top of the page.

This closes #96.

(Note this output still suffers from the double attempt issue reported in #100)

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

@conda/conda-libmamba-solver - ready for review! <3