Assertion without useful error: assembler.go

[roger-bosman-da]

https://github.com/DACH-NY/dpm/blob/a4c911026a7a657235fab6ecc8bfda9bfa42ed9f/pkg/assembler/assembler.go#L216 prints an unhelpful message without printing the commands compared. Error message should be extended or some kind of more expressive assertion (that prints a built-in more useful message) should be used