Prefix indexes are specified in characters while this error message
returns a limit in bytes. By adding the byte length in the message this
is makes it easier to understand.
This would deviate a little bit from the message in MySQL.
Before:
```
sql> CREATE TABLE t1(id int primary key, c1 text, key(c1(1000)));
ERROR: 1071 (42000): Specified key was too long; max key length is 3072 bytes
```
After:
```
sql> CREATE TABLE t1(id int primary key, c1 text, key(c1(1000)));
ERROR: 1071 (42000): Specified key was too long (4000 bytes); max key length is 3072 bytes
```
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