--exclude doesn't warn when excluding non-existing packages

[Nemo157]

Steps

→ cargo new foo
     Created binary (application) `foo` package
→ cd foo
→ cargo build --all --exclude bar
   Compiling foo v0.1.0 (/tmp/tmp.F4Gdb3Jlcr/foo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.37s

Possible Solution(s)
Emit a warning that bar is not a known package in this workspace.

Notes

Output of cargo version: cargo 1.34.0-nightly (245818076 2019-01-27)