Auto merge of #2480 - alexcrichton:ui, r=alexcrichton

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)