Feature request: Better diagnostics

[Volker-Weissmann]

Before "Go to Definition" will work, ccls needs to generate its .ccls-cache which can take a while.
If you try to "Go to Definition" before this has finished you might get the "No definition for ...". It would be nice if it would show
"No definition for ... , but this might change once the indexer has finished" instead (if the indexer has not finished yet).
Otherwise people might think they did not set ccls up correctly or think that there is a bug in ccls and spend a lot of time trying to find the problem.