Error message is confusing when changing opt levels

[steveklabnik]

I was trying out different optimization levels on a project. I started with s:

[profile.release]
opt-level = "s"

I wanted to compare it to 3, so I changed it:

[profile.release]
opt-level = "3"

This gives the error:

error: failed to parse manifest at `C:\Users\steve\tmp\oss101\Cargo.toml`

Caused by:
  must be an integer, `z`, or `s`, but found: 3 for key `profile.release.opt-level`

The error is that it's the string "3" not the integer 3, but the error message doesn't make this very clear.