parse{num}: Return the error from strconv.NumError

Returning the full error just adds extra noise to the error since it's
already clear that there was an error with the number. This now says
exactly what is wrong with the number.

parse.go

@@ -151,7 +151,7 @@ func parseInt(key, keytail string, values []string, target reflect.Value) error
 		return TypeError{
 			Key:  kpath(key, keytail),
 			Type: t,
-			Err:  err,
+			Err:  err.(*strconv.NumError).Err,
 		}
 	}
 
@@ -171,7 +171,7 @@ func parseUint(key, keytail string, values []string, target reflect.Value) error
 		return TypeError{
 			Key:  kpath(key, keytail),
 			Type: t,
-			Err:  err,
+			Err:  err.(*strconv.NumError).Err,
 		}
 	}
 
@@ -191,7 +191,7 @@ func parseFloat(key, keytail string, values []string, target reflect.Value) erro
 		return TypeError{
 			Key:  kpath(key, keytail),
 			Type: t,
-			Err:  err,
+			Err:  err.(*strconv.NumError).Err,
 		}
 	}