more informative error message on misformed commands

[Krastanov]

This is just a minor quality-of-life improvement for distracted users, but having slightly more descriptive error messages when a user on github uses a misformed @JuliaRegistrator register comment would be nice. Here is an example of an absentminded user repeatedly mistyping the @JuliaRegistrator register comments and wondering why it is not working QuantumSavory/QuantumClifford.jl@3ac1bf2

Specifically, @JuliaRegistrator misformed_command_string could give more specific explanation than just "unexpected error".