File not found while importing causes a SysError, not an EvalError,
which is not currently caught by the tab-completion handler. Ignoring
all SysErrors might seem "dangerous" but this is the tab-completion
handler, any exception being bubbled up from there causes unexpected
behavior (causes the whole repl to exit).
Fixes#340.
Change-Id: I643048a47935e77f582decc539d9e51bdb96c890