idmap2: move commands to Result<Unit>

Change the signature of the idmap2 commands (Create, Dump, ...) to
return Result<Unit> instead of bool. This removes the need to pass in an
ostream for error messages: instead, those messages are part of the
returned Result.

Consolidate error messages: texts in Error objects should not be
prefixed with "error:", that is the responsibility of the outer-most
caller (i.e. main()).

Test: make idmap2_tests
Change-Id: I074881b3d1982ea8f4be5752161ac74b14fcba95