Improve error message when put!-ting invalid data type into algorithm

I just spend too long deciphering the error message that comes up when trying to reconstruct a string (filename...) instead of an MPIFile. It would be nice to improve the error message here.

A potential solution would be to add a type to `put!`, e.g. `put!(algo, data::MPIFile)`, and add a `put!(algo, data)` fallback that outputs an explaining error message