util/Error: use std::exception_ptr instead of std::exception

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Same fix as in commit 1c904000

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -222,7 +222,7 @@ try {
 
 	return true;
 } catch (const std::exception &e) {
-	error.Set(e);
+	error.Set(std::current_exception());
 	return false;
 }