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

Adapt test cases.

regress/open_new_but_exists.test

diff --git a/regress/open_new_but_exists.test b/regress/open_new_but_exists.test
index 9fb0460..be3c9d1 100644
--- a/regress/open_new_but_exists.test
+++ b/regress/open_new_but_exists.test
@@ -4,3 +4,4 @@
 return 1
 file test.zip test.zip test.zip
 stdout opening `test.zip' returned error 10
+stderr 1 errors