Better error messages when TerminatedWorkerException is thrown

[MichaelHatherly]

Based on #276 the underlying error was difficult to debug since it was hidden. Improve the capture of errors from the worker processes that get killed.

[MichaelHatherly]

Some bits of info from having a look into this:

• would need to pipeline the Cmd in

  QuartoNotebookRunner.jl/src/Malt.jl

  Line 108 in ef1b93e

  proc = open(Cmd(cmd; detach = true, windows_hide = true), "w+")

  and capture stderr
• this is made more difficult since the likely place where code that would cause the process to get killed (via segfaulting perhaps) would be inside user code written in a cell. When that code is evaluated we have set stderr to a redirected pipe in IOCapture which captures that error (perhaps?) and thus it isn't forwarded to the stderr set in the server's pipeline in point one. Would likely need to also feed it through via passthrough keyword, but that sends it all to stdout (which is what Malt uses to communicate with the server..., uses a Socket, not stdout, that just gets used for the initial port number setting.