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".
(trunk r7905)

svn path=/main/branches/2.1.2/; revision=7941