Inconsistent formatting of Stdin

[mookid]

When formatting source code location errors, the formatting in the stdin case depends on the error level:

• case of a formatting error: <stdin>:line:column

  --> <stdin>:14:33
   |
14 |                         .takes_ value(true)
   |                                 ^^^^^ expected one of `)`, `,`, `.`, `::`, `?`, or an operator here

• case of an internal error: stdin:line:column

error[internal]: left behind trailing whitespace
  --> stdin:13:13:0
   |
13 |
   | ^^^^^^^^^^^^^^^^
   |

This is confusing for anyone that consumes the output of the command (rust-lang/rust-mode#365)

[topecongiro]

The first error looks like it is from the rustc parser, while the second one is from rustfmt itself.