Improve error handling on export

[Kris Deugau]

export_tiny doesn't handle many errors, so if it's passed a closed filehandle, the cache directory is variously missing, not a directory, or not read/writeable, or if there's some error creating and writing to the individual cache files, or the master file, it just keeps on going spitting warnings to stderr.

[Kris Deugau]

(In [566]) /trunk

Add I/O error-handling in export_tiny. There are probably better ways
to handle these than just die'ing, and in the long run adding Yet Another
Config Knob might be useful once we find a useful action other than stopping.

Closes #50, for now.