bookmarks: rely on exception for malformed lines
Since we already have an exception context open, for other thing, we can
simplify the code a bit and rely on exception handling for invalid lines.
Speed is not the main motivation for this changes. However as I'm in the middle
of benchmarking things we can see a small positive impact.
Before:
! wall 0.009358 comb 0.000000 user 0.000000 sys 0.000000 (best of 303)
After:
! wall 0.009173 comb 0.010000 user 0.010000 sys 0.000000 (best of 310)
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