Change tryopen to allow opening multiple files during one run.
Return number of errors at end.

Adapt test cases.

regress/open_nosuchfile.test

diff --git a/regress/open_nosuchfile.test b/regress/open_nosuchfile.test
index bd70a74..414cd5c 100644
--- a/regress/open_nosuchfile.test
+++ b/regress/open_nosuchfile.test
@@ -3,3 +3,4 @@
 args nosuchfile
 return 1
 stdout opening `nosuchfile' returned error 11/2
+stderr 1 errors