ERROR: Too many connectioons
Official documentation will rarely list all the things that could happen if advice or practices are not followed as the list would very quickly become far too long, unwieldy, and outdated. Instead it is much easier for the group — or individual — to state the recommended practice and move on, leaving all the different possible situations, conditions, and failures to sites like this one. By sharing the issue you encountered, other people can learn from your experience and hopefully resolve their issue.
With regards to your second question:

I couldn't get dump operation always make schema level lock -not to lock the whole table. So, how could this happen?

I am not quite sure what you’re asking but, when a backup of a table is being made, the table must be locked to ensure consistency. If records are modified or deleted during a backup process, then the backup may be wrong. If it is wrong, then it is unreliable. If it is unreliable then it cannot be trusted. If one backup cannot be trusted then …
You get the picture.
Generally what I will do with MySQL backups is pause replication, wait for any in-progress transactions to complete, lock the entire database, make a snapshot, unlock the database, then resume replication. This ensures the entire backup is accurate to a specific point in time.