Improve messages about json parsing errors

[TheMostDiligent]

The tool now prints the following error which does not tell which file exactly caused the problem:

terminate called after throwing an instance of 'nlohmann::detail::parse_error'
  what():  [json.exception.parse_error.101] parse error at line 62, column 13: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
terminate called recursively
Aborted (core dumped)
ninja: build stopped: subcommand failed.
ninja: build stopped: subcommand failed.

An error message should be clear about which file exactly failed to parse.