Error messages leak semicolons

[lihaoyi]

Currently, given this script

println("bazz")
rintln("foooo")
println("bar")

We get awkward semi-colons that appear at the start of the error message:

haoyi-mbp:~ haoyi$ amm Foo.sc
Foo.sc:2: not found: value rintln
;val res_1 = rintln("foooo")
             ^
Compilation Failed

This is probably an artifact of the way we preprocess the scripts in order to add the val res prefixes and stuff; we should be able to do it without adding these awkward semi-colons.