Unnamed variables in error messages

An unnamed variable (i.e., _) can be given a name in an error message. For example, consider the following code:
```
\func foo {A : \Type} (B : A -> \Type) (f : \Pi (a : A) -> B a) => 0

\func bar {A : \Type} (a : A) (B : A -> \Type) (b : B a) => foo B (\lam _ => b)
```
It produces the following error, which is confusing because the first 'a' is actually '_'
```
[ERROR] test.ard:37:79: Type mismatch
Expected type: B a
Actual type: B a
```
It is better to choose a name which clearly indicates that the variable was unnamed, like '_unnamed'. It is also unlikely that such a name will cause a clash of names as in the example above.

Reproduce the issue using the foo and bar example in the report and inspect the error-rendering path for unnamed variables. Done means the diagnostic distinguishes the unnamed variable, for example by displaying `_unnamed`, without creating a name clash.

Written by the indexing model from the issue text.