Improve error message when squiggles are due to no custom configuration provided

When a #include fails to resolve, and a custom configuration provider is explicitly set in c_cpp_properties.json, a warning that refers to information provided by the configurationProvider is emitted. i.e.:

#include errors detected based on information provided by the configurationProvider setting. IntelliSense features for this translation unit ({0}) will be provided by the Tag Parser.

#include errors detected based on information provided by the configurationProvider setting. Squiggles are disabled for this translation unit ({0}).

If the configuration provider was explicitly specified but not actually installed, or just was not set up to provide a custom configuration for that particular file, the error message seems incorrect. It could also check if a custom configuration was provided for that file. If not, the error message could be changes to not blame information provided by the configurationProvider.

Start with c_cpp_properties.json and trace how configurationProvider is handled when resolving a #include. Compare the message paths for an explicitly named provider that is unavailable or supplies no custom configuration for the file. Done means the warning no longer attributes the failure to provider information in those cases, with the existing messages preserved when custom configuration was provided.