* src/io-context-priv.h:
(struct _IOContext): Added error_occurred field.

* src/io-context.c
(gnumeric_io_context_new): Set error_occurred to FALSE by default.
(gnumeric_io_error_system, gnumeric_io_error_read,
gnumeric_io_error_save): Set error_occured to TRUE.
(gnumeric_io_error_unknown, gnumeric_io_clear_error,
gnumeric_io_error_occurred): New functions.

* src/io-context.h
(gnumeric_io_error_unknown, gnumeric_io_clear_error,
gnumeric_io_error_occurred): New functions.