Improving typesetting error messages for externalized TikZ pictures

[tueda]

For example:

! Package tikz Error: Sorry, the system call 'pdflatex -shell-escape -halt-on-e
rror -interaction=batchmode -jobname "tikzcache/doc-figure0" "\def\tikzexternal
realjob{doc}\input{doc}"' did NOT result in a usable output file 'tikzcache/doc
-figure0' (expected one of .pdf:.jpg:.jpeg:.png:). Please verify that you have
enabled system calls. For pdflatex, this is 'pdflatex -shell-escape'. Sometimes
 it is also named 'write 18' or something like that. Or maybe the command simpl
y failed? Error messages can be found in 'tikzcache/doc-figure0.log'. If you co
ntinue now, I'll try to typeset the picture.

In such a case, we can print the log file to show the actual typesetting errors.