Eliminate confusing false positive error messages

Eliminate confusing false positive error messages
for SRC_URI.syntax and digestentry.unused that are
reported when the SRC_URI for one or more ebuilds
can't be parsed for some reason. There's no point
in producing a false error here since the root
cause will produce a valid error elsewhere, such
as "SRC_URI.syntax" or "ebuild.sytax". Also,
update the docs for the "ebuild.syntax" error to
indicate that a digest verification failure can
trigger it.

svn path=/main/trunk/; revision=7905