zerocloud: avoid swift dependency for gettext (fixes #50)

Importing Gettext from Swift won't work out of the box. Swift relies
on a $SWIFT_LOCALEDIR environment variable being set in order for it
to find its compiled message catalogs (the .mo files). That variable
generally not set when running ZeroCloud.

Furthermore, we have begun marking our own strings for translation
with the _ function. This means that we need our own message catalog
if there is any hope of them being translated.
zerocloud: don't translate error messages

A server side application should probably not translate its strings
into the language of the server -- we don't know the language needed
by the client. It seems better to just keep the error messages fixed
in English.