Fix error handling of dblptex command

DBLP does not return a HTTP status code indicating that there is an
error but an error page instead. This is now checked by looking at the
content type. If something else than bibtex is returned, error out.

sok/management/commands/dblptex.py

@@ -1,6 +1,6 @@
 import requests
 
-from django.core.management.base import BaseCommand, CommandParser
+from django.core.management.base import BaseCommand, CommandError, CommandParser
 
 import sok.management.commands.dblpimport as dblp
 
@@ -17,4 +17,10 @@ class Command(BaseCommand):
 		url = f'https://dblp.uni-trier.de/rec/{key}.bib?param=0'
 		response = requests.get(url)
 		response.raise_for_status
+
+		# The status does not necessarily indicate success, but returns an error
+		# page instead.
+		if 'application/x-bibtex' not in response.headers['Content-Type']:
+			raise CommandError(url)
+
 		self.stdout.write(response.content.decode())