Bad error message when pyproject.toml for group doesn't exist

[konstin]

uv shows an incorrect error message when the pyproject.toml for a group doesn't exist, but somewhere above it there's a pyproject.toml that isn't usable (but should not have been checked in the first place):

[project]
name = "uv"
version = "0.8.17"

[tool.uv]
managed = false

$ uv pip compile --group does/not/exist/pyproject.toml:dev
error: Failed to read dependency groups from: does/not/exist/pyproject.toml
  Caused by: The project is marked as unmanaged: `/home/konsti/projects/foo`

Similarly, with an invalid pyproject.toml that's unrelated to out attempt at reading a dependency group:

[project]
name = "uv%%%"
version = "0.8.17"

$ uv pip compile --group does/not/exist/pyproject.toml:dev
error: Failed to read dependency groups from: does/not/exist/pyproject.toml
  Caused by: Failed to parse: `pyproject.toml`
  Caused by: TOML parse error at line 2, column 8
  |
2 | name = "uv%%%"
  |        ^^^^^^^
Not a valid package or extra name: "uv%%%". Names must start and end with a letter or digit and may only contain -, _, ., and alphanumeric characters.

We shouldn't read that pyproject.toml at all, we should error directly when we don't find does/not/exist/pyproject.toml.