Stop UNDEF exceptions

[Chris Warburton]

We're seeing output like this from Isabelle:

> ...
>    Global437573746f6d5a =
>   Global437573746f6d53 Global437573746f6d5a,
>   global70726f642f70726f705f33332e736d74326d756c74 Global437573746f6d5a
>    Global437573746f6d5a =
>   Global437573746f6d5a,
>   global697361706c616e6e65722f70726f705f30322e736d7432706c7573
>    Global437573746f6d5a Global437573746f6d5a =
>   Global437573746f6d5a,
>   END OUTPUT]:
>    string
> Exception trace - exception UNDEF raised (line 453 of "Isar/toplevel.ML")
> Exception trace - exception UNDEF raised (line 412 of "Isar/toplevel.ML")
> Exception trace - exception UNDEF raised (line 393 of "Isar/toplevel.ML")
> ### theory "ISACOSY"
> ### 571.591s elapsed time, 548.379s cpu time, 64.739s GC time
> val it = (): unit
> ML>

Annoyingly, this is spat to stdout rather than stderr. However, we're
still getting equations produced. We're not managing to parse anything
though, which may be due to the Isabelle process exiting with a failure
code, and 'pipefail' causing the whole pipeline to abort.

I'll try ignoring Isabelle errors for now, but it's better in the long
term to solve this.

This may be what the constr_trms stuff is for in commit 0b47d7cb3db4,
since that looks like it's saying 'ignore 'head([])', which we might
otherwise imagine would cause an 'UNDEF' exception.