Drop "invalid wide char string" message to debug level 3

Made fish unusable if the locale was C and non-ascii chars were in the
prompt.

See fish-shell#6584.

src/output.cpp

@@ -404,7 +404,7 @@ void outputter_t::writestr(const wchar_t *str) {
 
     size_t len = wcstombs(nullptr, str, 0);  // figure amount of space needed
     if (len == static_cast<size_t>(-1)) {
-        debug(1, L"Tried to print invalid wide character string");
+        debug(3, L"Tried to print invalid wide character string");
         return;
     }