util/Exception: add GetFullMessage(std::exception)

src/util/Exception.hxx

@@ -33,6 +33,15 @@
 #include <exception>
 #include <string>
 
+/**
+ * Obtain the full concatenated message of an exception and its nested
+ * chain.
+ */
+std::string
+GetFullMessage(const std::exception &e,
+	       const char *fallback="Unknown exception",
+	       const char *separator="; ") noexcept;
+
 /**
  * Extract the full message of a C++ exception, considering its nested
  * exceptions (if any).