Add missing err_cantopen.

As far as I can tell from the source control history, Halibut has
_never_ actually printed an error message on failure to open one of
its input files! The error message has existed all along, but was
never actually invoked. Ahem.

1 files changed, 3 insertions, 1 deletions

diff --git a/input.c b/input.c
index be59386..88ace8f 100644
--- a/input.c
+++ b/input.c
@@ -1664,7 +1664,9 @@ paragraph *read_input(input *in, indexdata *idx) {
 	    } else {
 		(*reader)(in);
 	    }
-	}
+	} else {
+            err_cantopen(in->filenames[in->currindex]);
+        }
 	in->currindex++;
     }