rustc, rustdoc: give nicer feedback when no arguments are given

[kud1ing]

A "how to use" information instead of the "upcall fail" messages would be nice:

$ rustc
error: No input filename given.
rust: upcall fail 'explicit failure', ../src/comp/driver/driver.rs:570
rust: domain main @0x10202aa00 root task failed

$ rustdoc
rust: upcall fail 'bounds check', ../src/rustdoc/rustdoc.rs:186
rust: domain main @0x10202aa00 root task failed

"cargo"'s feedback is quite good:

$ cargo
Usage: cargo <verb> [args...]
  init                                 Fetch default sources
  install [source/]package-name        Install by name
  install uuid:[source/]package-uuid   Install by uuid
  list [source]                        List packages
  search <name | '*'> [tags...]        Search packages
  sync                                 Sync all sources
  usage                                This