Made paths in error-output relative.

This greatly enhances readability when SALVE is invoked on a manifest,
since trees are localized collections of files. This alters the warnings,
but also the messages in SALVEExceptions.
Improved target writability check for file copy.

Instead of checking if the write flag is set on the parent dir, which only works
if the target file does not exist, attempts to open the file in write mode, and
closes it if there is an error.