qapi: Improve source file read error handling

qapi: Improve source file read error handling

qapi-gen.py crashes when it can't open the main schema file, and when
it can't read from any schema file.  Lazy.

Change QAPISchema.__init__() to take a file name instead of a file
object.  Move the open code from _include() to __init__(), so it's
used for the main schema file, too.

Move the read into the try for good measure, and rephrase the error
message.

Reporting open or read failure for the main schema file needs a
QAPISourceInfo representing "no source".  Make QAPISourceInfo cope
with fname=None.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-27-armbru@redhat.com>