Check for Java exceptions after void function calls

Exceptions encountered during `saveAsObjectFile` are being ignored as rJava does not check for exceptions after the method call for void functions [1]
This PR adds an explicit .jcheck to fix this problem.

Thanks to @hafen for pointing out this bug in #120 

[1] See https://github.com/s-u/rJava/blob/master/R/call.R#L112 -- the function returns before calling check https://github.com/s-u/rJava/blob/master/R/call.R#L139