Improve pool error detection

Goal of this task is to provide better error handling to pool.

Error message content 'Error during pool initialization' or 'Pool fails to create connection' depending on if this occurs during initialization or during pool lives.

Better connection error identification use 2 ways :

• Adding new 'error' event to pool.
• When pools doesn't have any connection, task reaching will throw connection error.

Error was a common "retrieve connection from pool timeout after 10000ms" for example. Now error message will be explicit depending on the issue