Log missing tikzmarks

[loopspace]

When the pic cs: syntax can't find a tikzmark then it uses a default coordinate; while this is okay, it should log the missing name to the log file. I keep getting caught out by this.

[loopspace]

Just adding a \message to the test for a missing tikzmark doesn't work as the \mathchoice bit works by seeing whether a tikzmark exists. So we'd need a conditional to switch on or off the warning.