Signed-off-by: Mattia Rizzolo <mattia@debian.org>

This is not simply fixing up spelling and grammar, but a full (ish)
review of English language strings and translation structure. This lays
the groundwork for a more formal translation round at a later date.

Some of the phrasing will inevitably be coloured by en_GB preference,
but I don't feel any shame about that.

Chris Lamb authored Sep 09, 2017 and Jonathan McDowell committed Nov 06, 2017

This fixes somewhat ugly grammar of, for example:

  "You are currently Debian Developer, uploading and you cannot
   become Debian Developer, non-uploading."

to

  "You are currently a Debian Developer, uploading and you cannot
   become a Debian Developer, non-uploading."

Thanks to Chris West (Faux) <debian@goeswhere.com> for pointing
this out on IRC.

Closes: #875128

The checks in /wizard/process for the desired target status did not
gracefully handle an invalid destination (in particular dd is not valid,
it is dd_u). Fail with an invalid error message rather than a Python
backtrace.