Tweak UI for warnings and errors

Tweak UI for warnings and errors

Right now Cargo prints out errors justified like all other status messages, but
this can look odd without coloration:

```
   error some error message
```

Instead, this commit changes both warnings and errors to use the same style of:

```
error: some error message
warning: some warning message
```

Additionally, warnings now only print out "warning" in yellow instead of the
entire message (like errors do)