Friendly error message when trying to launch addin picker and vsc.rstudioapi = FALSE

[MilesMcBain]

What problem did you solve?

In the case that the user has not set the option vsc.rstudioapi = TRUE the addin picker gives an unhelpful error message. This PR adds an error message that directs the user to set the option to TRUE.

A root cause of #439 was the confusing error.

(If you do not have screenshot) How can I check this pull request?

In your .Rprofile set vsc.rstudioapi = FALSE. Start a new R terminal in VSCode and run the R: Launch RStudio Addin command.

An error should appear:

Command 'R: Launch RStudio Addin' resulted in an error (Could not find list of installed addins. options(vsc.rstudioapi = TRUE) must be set in your .Rprofile to use RStudio Addins)

[renkun-ken]

LGTM