Unify the format of the write_error_msg

I had previously updated the text accompanying errors in command.hpp (which gets called for errors during initialization) but not the accompanying text in base_hamiltonian.hpp (which gets called during sampling).  Only constant text has been modified -- no implementation details are touched so no tests should be affected.

Not an immediate priority but would be nice to add before 2.0 and could easily be wrapped up into another request to avoid another round of testing.