Add better error message when failing to read large files.

[nylander]

Trying to read large files (with very long lines, see issue #109), the user sees the error message "Maximum allowed length of a token is 99990".

It would be more helpful to give a more informative error message, perhaps also saying that a workaround is to use an interleaved format (or even apply line breaks in the sequence?).

Another alternative is to actually programmatically address the hard limit on "max length of a token".

This issue could possibly be coupled with issue #67.