Fix catagorization of guideline severties.

[PhilippSalvisberg]

In the talk Fighting Bad PL/SQL & SQL I categorised the violations of the following guidelines as bugs:

Id	Title	Severity in 4.2
G-1080	Avoid using the same expression on both sides of a relational comparison operator or a logical operator.	Minor
G-2150	Avoid comparisons with NULL value, consider using IS [NOT] NULL.	Blocker
G-3185	Never use ROWNUM at the same query level as ORDER BY.	Major
G-4230	Always use a COALESCE instead of a NVL command, if parameter 2 of the NVL function is a function call or a SELECT statement.	Critical
G-4250	Avoid using identical conditions in different branches of the same IF or CASE statement.	Major
G-5080	Always use FORMAT_ERROR_BACKTRACE when using FORMAT_ERROR_STACK or SQLERRM.	Minor
G-7810	Never use SQL inside PL/SQL to read sequence numbers (or SYSDATE)	Major
G-9010	Always use a format model in string to date/time conversion functions.	Major

Bugs are a blocker IMO. Hence all these guidelines should be blocker according to Severity of a rule. G-4230 is an exception since the violation does not lead to a wrong result, just to slower execution. Hence "critical" is an acceptable severity in this case. For a similar reason, G-5080, G-7810 should be changed to "critical" and not "blocker".

It's probably a good idea to check the severity of all guidelines and amend them if necessary.