inform user if tsconfig.json is not found

[abenhamdine]

I ran into the pb described in #213, and, indeed, setting "typescript_tsdk" in Package settings in order to locate the ts lib has fixed this pb.

I spent quite an hour to investigate the issue, and I think most newcomers are likely to run into this (because they will probably install last version of ts globally, as I did), so it should be a good think :

• to document this more obviously (the setting is documented in the readme, but not the fact that it can solve this issue).
• and to inform the user if no tsconfig.json was found by the compiler