Don’t use type Error = ! for target-dependant TryFrom impls.

Instead, expose apparently-fallible conversions in cases where
the implementation happens to be infallible for a given target.

Having an associated type / return type in a public API change
based on the target is a portability hazard.

Author: SimonSapin

Diff for: src/libcore/num/mod.rs

@@ -3681,7 +3681,7 @@ macro_rules! try_from_unbounded {
     ($source:ty, $($target:ty),*) => {$(
         #[unstable(feature = "try_from", issue = "33417")]
         impl TryFrom<$source> for $target {
-            type Error = !;
+            type Error = TryFromIntError;
 
             #[inline]
             fn try_from(value: $source) -> Result<Self, Self::Error> {