Give better error message if dictionary path is not writable

[Sharpie]

Rather than relying on the unspecific "cannot create lock file" message Produced by Filehash, there should be a clear error message if the directory used to store the metrics dictionary is not writable.

Also, investigate the behavior of paths containing UTF-8 symbols as noted in #66.

[krlmlr]

Works for me (the tmp directory is not writeable).

Creating new TikZ metrics dictionary in:
    tmp/ä
Error in doFUN(dbName) : unable to create database file 'tmp/ä'
In addition: Warning message:
In file.create(dbName) :
  cannot create file 'tmp/ä', reason 'Permission denied'

Also #66 doesn't occur in Ubuntu 13.04.