Downgrade internal errors to reporter warnings in file/directory BiFs

This allows Zeek execution to continue gracefully in the presence of such
errors, particularly at zeek_init() time. Includes a tweak to expand the
bifs.directory_operations test to check continuation after errors.

Resolves #3595.

testing/btest/Baseline/bifs.directory_operations/out

@@ -9,3 +9,4 @@ F
 F
 F
 F
+Shutting down.

testing/btest/bifs/directory_operations.zeek

@@ -4,7 +4,7 @@
 
 event zeek_init()
 	{
-	# Test succesful operations...
+	# Test successful operations...
 	print mkdir("testdir");
 	print mkdir("testdir");
 	local a = open("testdir/testfile");
@@ -14,7 +14,7 @@ event zeek_init()
 	print unlink("testdir2/testfile2");
 	print rmdir("testdir2");
 
-
+	# ... and failing ones.
 	print unlink("nonexisting");
 	print rename("a", "b");
 	print rmdir("nonexisting");
@@ -22,3 +22,9 @@ event zeek_init()
 	close(a);
 	print mkdir("testfile");
 	}
+
+event zeek_done()
+	{
+	# Only reached when above failures don't cause Zeek to exit.
+	print "Shutting down.";
+	}