NBConvert 5.5 final PRs

[Matthew Seal]

Hello Jupyter devs and users.

We're close to getting jupyter/nbconvert 5.5 released with a bunch of improvements. But we could use some help with the last few issues to get the release going.

There's several open issues that might have simple solutions or fixes we could get in, or stage for 5.5.1 later on. I've tagged "Help Wanted" on several issues in https://github.com/jupyter/nbconvert/issues that could use some broader opinions / knowledge bases to help answer or investigate.

The following PRs are targeted for 5.5 and could use some additional eyes if folks have the time and know the repo well enough.

#986 Fixed pdf export path and naming failures (has regex and lots of path rewriting that could use a +1)

    - Also overlaps / covers # 947 Return an error if latex does.

#971 Added test for kernel death (better test for earlier PRs that got merged)

#919 exclude header config (needs discussion / opinions)

#982 Added tests for each branch in execute's run_cell method (longer running thread but attached test PR gives a lot more confidence to the equivalence of execution, eyes here appreciated as the code is critical to all nbconvert operations)

    - Precursor PR for #905 Refactored execute preprocessor to have a process_message function

Thanks,

Matt