Catch GpgME error when signing.

Listen to GpgME if an error occured when attempting to sign, and
abort if that is the case.

src/fmail.c

@@ -349,13 +349,16 @@ sign_stream(gpgme_ctx_t ctx, FILE *in, FILE *out)
 {
     gpgme_data_t gin, gout;
     gpgme_sign_result_t result;
+    gpgme_error_t gerr;
     char boundary[32], buffer[512];
     int n;
 
     gpgme_data_new_from_stream(&gin, in);
     gpgme_data_new(&gout);
 
-    gpgme_op_sign(ctx, gin, gout, GPGME_SIG_MODE_DETACH);
+    gerr = gpgme_op_sign(ctx, gin, gout, GPGME_SIG_MODE_DETACH);
+    if ( gerr != GPG_ERR_NO_ERROR )
+	errx(EXIT_FAILURE, "signing failed: %s", gpgme_strerror(gerr));
     result = gpgme_op_sign_result(ctx);
 
     generate_boundary(boundary, sizeof(boundary));