Dump std::exception.what() when possible

When doing the panic on unhandled exceptions, try and grab the
.what() pointer and dump it as part of the termination info.
Makes it easy to see mem errors (std::bad_alloc) or std::runtime_error
strings.

cores/esp8266/core_esp8266_main.cpp

@@ -150,14 +150,31 @@ static void do_global_ctors(void) {
         (*--p)();
 }
 
-extern "C" { extern void __unhandled_exception(); }
+extern "C" {
+extern void __unhandled_exception(const char *str);
 
+static void  __unhandled_exception_cpp()
+{
+    static bool terminating;
+    if (terminating)
+        abort();
+    terminating = true;
+    /* Use a trick from vterminate.cc to get any std::exception what() */
+    try {
+        __throw_exception_again;
+    } catch (const std::exception& e) {
+        __unhandled_exception( e.what() );
+    } catch (...) {
+        __unhandled_exception( "" );
+    }
+}
 
+}
 
 void init_done() {
     system_set_os_print(1);
     gdb_init();
-    std::set_terminate(__unhandled_exception);
+    std::set_terminate(__unhandled_exception_cpp);
     do_global_ctors();
     esp_schedule();
 }

cores/esp8266/core_esp8266_postmortem.c

@@ -40,7 +40,7 @@ static const char* s_panic_func = 0;
 static const char* s_panic_what = 0;
 
 static bool s_abort_called = false;
-static bool s_unhandled_exception = false;
+static const char* s_unhandled_exception = NULL;
 
 void abort() __attribute__((noreturn));
 static void uart_write_char_d(char c);
@@ -121,7 +121,7 @@ void __wrap_system_restart_local() {
         ets_putc('\n');
     }
     else if (s_unhandled_exception) {
-        ets_printf_P("\nUnhandled exception\n");
+        ets_printf_P("\nUnhandled exception: %s\n", s_unhandled_exception);
     }
     else if (s_abort_called) {
         ets_printf_P("\nAbort called\n");
@@ -237,8 +237,8 @@ void abort() {
     raise_exception();
 }
 
-void __unhandled_exception() {
-    s_unhandled_exception = true;
+void __unhandled_exception(const char *str) {
+    s_unhandled_exception = str;
     raise_exception();
 }