Make BinsBuildError enum non-exhaustive

Once the `#[non_exhaustive]` attribute is stable, we should replace
the hidden enum variant with that attribute on the enum.

Author: jturner314

src/histogram/errors.rs

@@ -25,6 +25,8 @@ pub enum BinsBuildError {
     EmptyInput,
     /// The strategy for computing appropriate bins failed.
     Strategy,
+    #[doc(hidden)]
+    __NonExhaustive,
 }
 
 impl BinsBuildError {