Updated all cases of exit() to return a valid code

Specific codes are as follows, but can easily be changed if a different order/breakdown makes more sense:

- 0: Success; everything executed as planned
- 1: Configuration Error; something is wrong with/in the configuration file(s)
- 2: Class Not Found; what it says
- 3: Driver Method Unsupported; the method you're trying to use on a Driver doesn't exist
- 4: File Not Found; 404 error
- 5: Database Error; something is broken in the database somewhere
- 6: Invalid Input; the user attempted to submit a request with invlaid characters in 1+ key names
7 through 26 are reserved for future use
- 27: Generic Error; generated by show_error() when the status code is >= 100
28 through 127 are errors generated by user applications, normally by using show_error() with a status code below 100
128 through 254 should not be used by applications, as they are reserved by system-level functions
- 255: PHP Fatal Error; automatically generated by PHP for fatal errors, and therefore not allowed for our use

Status codes below 100 are shifted up by 28 to place them in the user error range.  It may make more sense to have these codes
left alone and instead shift the CI errors into the 101 through 127 space, but that's not what I opted for here.

It would probably also be a good idea to replace the hard-coded numbers with constants or some such, but I was in a bit of a
hurry when I made these changes, so I didn't look around for the best place to do this.  With proper guidance, I could
easily amend this commit with another that uses such constant values.

Signed-off-by: Daniel Hunsaker <danhunsaker@gmail.com>
Set transaction enabled flag default to TRUE
DB_driver's trans_complete exception fix
Issue #2086 Session_cookie's _update_db not guaranteed to update
Merge pull request #2267 from dionysiosarvanitis/fix/_update_db

Issue #2086 Session_cookie's _update_db not guaranteed to update
Merge pull request #2265 from dionysiosarvanitis/fix/trans_enabled

Set transaction enabled flag default to TRUE
Remove CI_DB_pdo_driver::

Improving on PR #2265, the property is inherited with the same value and doesn't need to be set.
Update user_guide_src/source/tutorial/news_section.rst

Clarified a part of the tutorial so it's obvious the code should be replaced/update rather than added.
Fix for issue #2236
Fix Session tests
[ci skip] Remove a changelog line for a non-existent change
Manually apply PR #2234
Disable autoloader call from class_exists() occurences to improve performance

Note: The Driver libary tests seem to depend on that, so one occurence in CI_Loader is left until we resolve that.
DB_result tests seem to also depend on autoloading via the class_exists() checks ...
Fix #2273
Merge pull request #2266 from dionysiosarvanitis/fix/trans_complete

DB_driver's trans_complete exception fix
Merge pull request #2274 from ckdarby/patch-1

Fix #2273
Add CI_Utf8::convert_to_utf8() test
Some miscellaneous tests
CI_Output [set/append/get]_output() tests