Give meaningful error messages when parsing LPs

Currently, parsing errors for LP files result in CoinError exceptions being thrown, without any further information; it does not even give a stack frame or even just a report of the currently executing method. This makes finding out why a specific LP file cannot be parsed take hours instead of seconds (at least if no debugger is attached).

I would suggest to:

- add more meaningful error messages than `CoinError`
- add line and column numbers to indicate where the problem occurred.

(Opening this issue as requested in coin-or/Clp#146.)

The payload names no source files, tests, or parser entry points. Start by locating the LP-file parsing code and existing CoinError paths; done means parse failures provide meaningful context and report the line and column where the problem occurred.

Written by the indexing model from the issue text.