try to improve syntax error line numbers. fixes #4844

also reword the LoadError message. "at" vs. "in" was not helpful, so
instead make it clear that this is during loading of a file, and that the
line number refers to the start of the expression that caused the error,
since in many cases that's the best we can do.
Merge branch 'master' of github.com:JuliaLang/julia