feat: add location information to markdown parsing errors
Description
This makes error messages about problems in the markdown source a bit more useful by adding location in the source file. This doesn't do anything to errors that don't arise from markdown files, e.g., parsing of bindings files (#96).
Fixes #16 (closed)
Checklist
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You have read, and followed the guidance in, the CONTRIBUTING guide -
The MR content has run through a pipeline, or at least has pipelines enabled. -
There are no licence concerns with this contribution (The README clearly states Subplot is MIT-0)