[FEAT]: Introduce new error message for packages without `mainProgram`

Is your feature request related to a problem?

Some packages have multiple binaries with different purposes therefore setting mainProgram is not desirable.

When mainProgram is not set for these packages nix run errors out with

error: unable to execute '/nix/store/abc/bin/xyz': No such file or directory

Proposed solution

A new error for packages that don't have the mainProgram attribute set, something like

error: in 'abc': 'mainProgram' isn't set

Alternative solutions

An option to set mainProgram to null for packages that desire to do so, leaving the default behavior of mainProgram = pname; intact. An error like

error: 'abc' doesn't have a default binary

would be better in this case.

Additional context

https://github.com/NixOS/nixpkgs/issues/486543

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

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