It is actually a good thing that it is implemented that way and protects you from various further problems.
Implementing such a feature would make your shop more vulnerable for information gathering.
Imagine the following attack vector:
- Get a (very long) list of e-mail addresses
- Check which e-mail addresses exist in your shop by using the output of the page (success/error)
- Use all successful (thus: registered) e-mail addresses and start brute forcing the passwords
Not only will this give a possible attacker the information which e-mail addresses are registered, it will enable him to guess passwords by using brute force.
If your shop does not provide this information in the frontend, it will be much harder to find a correct combination of e-mail address and password as you cannot rely on success/error information provided by the shop.
This might harm your shop in an additional way as brute force attacks may result in denial of service (DoS) due to the possible high load in your shop.