shell/math: deconvolute and explain ?: handling. Give better error message

function                                             old     new   delta
arith_apply                                         1271    1283     +12

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/shell/hush_test/hush-arith/arith.right b/shell/hush_test/hush-arith/arith.right
index fd4ea8e..8a201fb 100644
--- a/shell/hush_test/hush-arith/arith.right
+++ b/shell/hush_test/hush-arith/arith.right
@@ -81,7 +81,7 @@
 hush: arithmetic syntax error
 16 16
 hush: arithmetic syntax error
-hush: arithmetic syntax error
+hush: malformed ?: operator
 hush: arithmetic syntax error
 9 9
 hush: arithmetic syntax error