Make import_instruments fail gracefully

[RaymondBalise]

If an API key has been revoked TR gives a useless/wrong message. Add test:

check_project_exists <- function(redcap_uri, token) {
  result <- redcap_project_info_read(
    redcap_uri = redcap_uri,
    token      = token,
    verbose    = FALSE
  )
  result$success
}

# Usage
uri   <- "https://redcap.yourinstitution.edu/api/"
token <- "YOUR_32_CHARACTER_TOKEN"

check_project_exists(uri, token)