feature: Add more robust error handling

[shejialuo]

Now when loading the corresponding clients, it uses a try-except to make sure that the specified client in the configuration file has been installed by the user. However, there are some tricky things. The code uses c.load() to load a distribution. However, the c.load() itself may fail which causes exception "Specified client not found". Although users have already installed some specified client, c.load() could fail in the following situations:

• The explicit dependency error (the client dependency doesn't require).
• The implicit dependency (it is more tricky to deal).

However, the code just provides exception "Specified client not found" which is not clear for users. That's why there are many issues which talks about this exception (#105 #113 #102)

So this patch aims at providing more robust error handling which could make users understand what really happens to make the users life easier.

[milkice233]

Thanks for your contributions! I'm afraid the review may take some time because I'm just so busy (or a little bit procrastinating I admit) that I even have no time to refactor the code. I'll get back longer.

[shejialuo]

@milkice233 Hello, I am sorry to trouble you. Is there a feedback for this PR.