Problem: When `nvim_set_keymap` tries to overwrite a `<unique>` mapping,
it throws an error even when called in `pcall`.
Solution: src/nvim/mapping.c:buf_do_map no longer calls `semsg`. Its
callers now decide whether to ignore the error, or use
`semsg` (not caught)/`api_set_error` (caught by `pcall`).
(cherry picked from commit ec18ebcb41)
5fc6bd6454