Improve error messages for FatalLexingError

[Drvi]

These can happen when the buffer is too small to fit the largest row in the input, or when we misconfigured the lexer, or if the file is not parseable. It's hard to know which of these is the problem, but we should try and do a bit of analysis on the current chunk (like looking for how strings are escaped and comparing it with our escape character, look if there are any newlines in the chunk...) to give our best guess (and evidence for it)