Adding a package adds it to pixi.toml even if it failed

[AlexisTM]

Checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pixi, using pixi --version.

Reproducible example

pixi init
pixi add colcon # results as: Cannot solve the request because of: No candidates were found for colcon *.
pixi add cmake # results as: Cannot solve the request because of: No candidates were found for colcon *. 

Issue description

When adding a package, we may have had a typo or maybe we added a non-existent package.
Regardless, this package will be added to the toml and will make it fail for all further calls.

Expected behavior

If the package doesn't exist, it shouldn't be added to the toml and report an error the package doesn't exist.