Propagate exceptions raised from #to_int in Time::at offset parsing

[lopopolo]

we need to short circuit return from implicitly_convert_to_int here to handle this case:

[3.1.2] > class A; def to_int; raise 'bam'; end; end
=> :to_int
[3.1.2] > Time.at(12, in: A.new)
(irb):6:in `to_int': bam (RuntimeError)
        from <internal:timev>:274:in `at'
        from (irb):7:in `<main>'
        from /usr/local/var/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/irb-1.4.1/exe/irb:11:in `<top (required)>'
        from /usr/local/var/rbenv/versions/3.1.2/bin/irb:25:in `load'
        from /usr/local/var/rbenv/versions/3.1.2/bin/irb:25:in `<main>'

Originally posted by @lopopolo in #1956 (comment)