Improved error messages if invalid character is used

[timcadman]

Observed behaviour

Currently if you try to pass an invalid symbol through the R parser you receive an error something like this:

$sim1
[1] "Syntax error at '\"'"

$sim2
[1] "Syntax error at '\"'"

$sim3
[1] "Syntax error at '\"'"

Expected behaviour

More informative would be something like:
"You have tried to pass character 'x', however this character is not permitted by the R parser. See etc etc"