Handle errors in getAliases.R

[csaybar]

What problem did you solve?

When an aliases.rds file in one of your installed packages is corrupt or improperly formatted, it can cause the documentation to fail every time it is requested, resulting in an error message that can be quite frustrating.

(x) Failed to get list of R functions. Make sure that jsonlite is installed and r.rpath.linux points to a valid R executable.
(x) Error in readRDS(filename) : error reading from connection Calls: lapply -> FUN -> as.list -> readRDS Execution halted

This PR adds a more informative message to the users.

[csaybar]

I am not sure why I cannot upload images on GH, but I am able to upload PDF files.
bug01.pdf

[renkun-ken]

Thanks for trying to fix the error handling. Unfortunately, the warning does not go to the user. I rewrite the code to make it work.