WL#4810: Reserve error codes for GA releases

The problem is that every time a new error message is added to a GA version of 
MySQL, the code (number) of the error messages only present in later versions will 
all change as the error added to 5.1 must have the same code (number) in every 
release.

In a nutshell, the consequence is a stream of ever changing error codes in 6.0 
every time we add a new error to 5.1, which makes developer's work hard as a lot 
of tests have to be updated and users can't depend upon the error codes of non-GA 
releases (eg: broken SP handlers, etc).

Nonetheless, we could easily fix this problem by reserving, let's say, 25 error 
messages for each MySQL release.

See also WL#4244 "Error codes supporting multiple releases".

In order to reserve error messages for a previous release, it's necessary to:

1) Modify extra/comp_err.c

It's necessary to modify the start-error-number command to also reset the error 
code counter if a new "start error number" is specified.

2) Modify sql/share/errmsg.txt

In later MySQL versions with start-error-number tags to reserve error code ranges.

For example:

MySQL 5.1 sql/share/errmsg.txt:

start-error-number 1000

ER_NO
  "No"

ER_YES
   "Yes"

MySQL 6.0 sql/share/errmsg.txt:

start-error-number 1000

ER_NO # 1000
  "No"

ER_YES # 1001
   "Yes"

start-error-number 1050

ER_MAYBE # 1050
   "Maybe"

This effectively reservers 50 error codes for MySQL 5.1.