Report error in tilde expansion

[magicant]

The current implementation ignores any errors in tilde expansion. The expansion should fail if there is an error so that the user can notice the error more easily.

For the error message to include an annotated source code snippet, the WordUnit::Tilde variant needs to carry its originating location, and that requires the TextUnit::Literal variant to have a location in turn.

• Let TextUnit::Literal carry its location
• Let WordUnit::Tilde carry its location
• Let tilde expansion fail on error