Change the `joinError` functions to have stronger guarantees when used:
* Change the template signature to `RangeOf` to ensure that the
container has `llvm::Error`s.
* Check that the size of the passed container is actually positive
before deferencing `.begin()`.
Add the `revng::cantFail` function which allows to wrap functions with
`std::error_code` and check their result. This is analogous to
`llvm::cantFail` for `llvm::Error`.
Giacomo Vercesi