test that each email 'told' exists in user's keyring. Closes #176

src/commands/git_secret_tell.sh

@@ -64,6 +64,20 @@ function tell {
     _abort "you must provide at least one email address."
   fi
 
+  local gpg_uids
+  gpg_uids=$(_get_users_in_gpg_keyring)
+  for email in "${emails[@]}"; do
+    local email_ok=0
+    for uid in $gpg_uids; do
+        if [[ "$uid" == "$email" ]]; then
+            email_ok=1
+        fi
+    done
+    if [[ "$email_ok" == 0 ]]; then
+      _abort "email not found in gpg keyring: $email"
+    fi
+  done
+
   local start_key_cnt
   start_key_cnt=$(get_gpg_key_count)
   for email in "${emails[@]}"; do