Improve the las reader's file signature error

Catch the las header's invalid file signature error and improve the
error message, mostly by including the file name. In order to provide
the filename under more circumstances, set the "filename" option when
using the filename-only las::Reader constructor.

Fixes #277.
Merge pull request #351 from gadomski/issue/277-inval-file-sig

Improve the las reader's file signature error