Custom config provider extension users should not see the message "Please update your includePath."

[sherryyshi]

Currently when an include search path does not resolve, the message displayed to users is "#include error detected. Please update your includePath." This message is not helpful for custom configuration providers, since we don't want the users to update the includePath themselves, which would override the results the config provider provides.

In our scenario, the user would see this every time at start up since it takes a few seconds for the makefile parsing to complete. In this case, we especially don't want the users to be updating any configurations on their own.

[sean-mcmanus]

We have a custom/different message for compile_commands -- looks like a custom message for custom config providers was just an oversight.

This is actually a bug with the C/C++ extension and not the API (https://github.com/microsoft/vscode-cpptools/issues) -- not sure if we should move it.