qapi: Change frontend error messages to start with lower case

qapi: Change frontend error messages to start with lower case

Starting error messages with a capital letter complicates things when
text can get interpolated both at the beginning and in the middle of
an error message.  The next patch will do that.  Switch to lower case
to keep it simpler.

For what it's worth, the GNU Coding Standards advise the message
"should not begin with a capital letter when it follows a program name
and/or file name, because that isn’t the beginning of a sentence. (The
sentence conceptually starts at the beginning of the line.)"

While there, avoid breaking lines containing multiple arguments in the
middle of an argument.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-7-armbru@redhat.com>