rustup doc should deal with absent doc instead of silently failing #901
Description
On a new install of rustup, running rustup doc fails silently if the documentation is not installed: the browser does not open, and there is no error message.
Would it be OK to add an error message, or to automatically run rustup component add rust-docs ?
This is tangentially related to #891 — if the doc is installed by default, this issue would be moot.