results: Bail out gracefully when profile data is missing

[chiruu12]

Closes #1459.

get_profile indexed _profiles directly and then called .encode() on the result, so a benchmark with no stored profile raised KeyError, and one stored as None raised AttributeError. add_result only writes the entry when the profile is truthy, so both shapes are reachable.

has_profile('absent')  -> None
get_profile('absent')  -> KeyError: 'time_absent'
get_profile(None-valued) -> AttributeError: 'NoneType' object has no attribute 'encode'

It now raises UserError, which main.py already turns into a logged error and a non-zero exit rather than a traceback. Of the three call sites, two guard with has_profile first, and the third is asv profile right after a profiling run, which is the one that surfaced this.

Also corrects the docstring, which promised pstats.Stats but returns raw bytes. get_profile_stats has been the Stats-returning method since #1253.

Added a test covering both shapes. It fails on main with KeyError and passes here.

pytest test/test_profile.py test/test_results.py passes, 11 tests. pre-commit run --files passes on all changed files.

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