ircd::util: Add util to get what() from std::exception_ptr.

include/ircd/util/util.h

@@ -472,5 +472,26 @@ pointers(input_container&& ic,
 }
 
 
+/// Get what() from exception_ptr
+///
+inline ircd::string_view
+what(const std::exception_ptr &eptr)
+try
+{
+	if(likely(eptr))
+		std::rethrow_exception(eptr);
+
+	return {};
+}
+catch(const std::exception &e)
+{
+	return e.what();
+}
+catch(...)
+{
+	return {};
+}
+
+
 } // namespace util
 } // namespace ircd