Improve Location for "type variable is already in scope" error

[jfecher]

Minimal repro for error:

type Foo t t = x: t

Current error:

$ ante t.an --no-color
t.an:1:1	error: Type variable 't' is already in scope
type Foo t t = x: t
^^^^^^^^^^^^^^^^^^^

Desired error:

$ ante t.an --no-color
t.an:1:12	error: Type variable 't' is already in scope
type Foo t t = x: t
           ^

t.an:1:10	note: 't' was previously declared here
type Foo t t = x: t
         ^