feat: improve error and feedback when target does not exist

[tdejager]

Fixes issue: #1928

Even though what was happening there was intended behavior, there was an expect and just general unclearness. Although I feel the UX could be improved further, this improves the feedback at least. Now giving you the error:

pixd remove flask --platform linux-64                                                                                                                                                                                                                                                                  ─╯
  × failed to remove dependency
  ╰─▶ No target for feature `default` found on platform `linux-64`
  help: Expected target for `default`, e.g.: `[target.linux-64.dependencies]

Given this toml:

[project]
name = "bug"
channels = ["conda-forge"]
platforms = ["linux-64"]

[dependencies]
flask = ">=3.0.3,<4"

When calling the equivalent command from the issue. This should be enough to improve the situation for now I think.

[nichmor]

super nice!