Don't crash fast-import if the marks cannot be exported.

Apparently fast-import used to die a horrible death if we
were unable to open the marks file for output.  This is
slightly less than ideal, especially now that we dump
the marks as part of the `checkpoint` command.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -1367,8 +1367,12 @@ static void dump_marks(void)
 	if (mark_file)
 	{
 		FILE *f = fopen(mark_file, "w");
-		dump_marks_helper(f, 0, marks);
-		fclose(f);
+		if (f) {
+			dump_marks_helper(f, 0, marks);
+			fclose(f);
+		} else
+			failure |= error("Unable to write marks file %s: %s",
+				mark_file, strerror(errno));
 	}
 }