Add an informative error message about missing TeX distribution

[mariusbarth]

I wonder whether it might be a good idea to check, maybe on package startup, whether tinytex (i.e., the R package and the TeX distribution) are installed. I recently ran into the problem that I only got the error message apa6.cls was not found, and this was too cryptic to figure immediately that I just had not installed tinytex. Things will get complicated if users have another TeX distribution installed, but I think it is worth considering to add a test and an informative error message.