Possible fix for issue 261 ?

Hi

This is my very first use of gitHub so please forgive (but do correct) any protocol breaches I have blundered into.

I have had a go at correcting what I think is the source of the problem in issue 261 https://github.com/klutometis/roxygen/issues/261
namely that 
```top <- eval(help_call)```  
in roxygen2::get_rd raises an error condition if help cannot find the documentation requested (as well as returning a ``top''' which has length == 0.)

This is also my first attempt at using tryCatch and conditions, so I'm sure the code can be made more elegant.  It does seem to work on my toy example (in the sense that devtools::document() now completes running and generates the documentation I expect.)

I'm not sure what consequences this suggested change might have for elsewhere in the roxygen2 code though.  In particular if a user were to attempt to explicitly ```@inheritParams package::function``` from a package that was not being worked on by roxygen2 at that point in time, and generated a "Could not find ..." style error message, I'm unsure what would happen downstream in the roxygenising process.  (To be honest I'm not even sure if such an ```@inheritParams``` tag would end up in this segment of code.)

Hoping this is useful.

Geoff