Give better error messages when parsing fails
Give better error messages when parsing fails

An attempt to give slightly more helpful error messages when we have a problem
parsing an event schema file (in particular, which file contains the problem,
and which property within it).

A bit of light refactoring to make it tractable.

matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc