We have Error.cpp/.h which contains some code for working with error codes.
In fact we use Error/Expected<> almost everywhere already and we can get rid
of these files.
Note: a few places in the code used readobj specific error codes,
e.g. return readobj_error::unknown_symbol. But these codes are never really used,
i.e. the code checks the fact of a success/error call only.
So I've changes them to return inconvertibleErrorCode() for now.
It seems that these places probably should be converted to use Error/Expected<>.
Rebased on top of D86771.