Don't swallow exceptions in transctional statements

The MySQL connection adapater swallows all StandardError exceptions,
which includes Mysql::Error and Mysql2::Error. The comment in the
exception clause claims errors thrown here indicate that transactions
aren't supported by the server but that isn't necessarily true. It's
possible the MySQL server has gone away and swallowing a failed commit
may let the application return a successful response when the data has
not been saved. Also, replication libraries like Galera require that the
application handle exceptions thrown at BEGIN/COMMIT.

I'm unable to determine what version of MySQL threw an exception for
transactional statements. I tried as far back as 3.23.49 with InnoDB
disabled but BEGIN & COMMIT statements do not throw an error. If there's
a real case for this logic to continue, we could instead push this
behavior into a configuration setting.

The exception swallowing has been there since the beginning:
db045dbbf60b53dbe013ef25554fd013baf88134
Update security.md

Hi Guys 
I was reading through this guide last night and noticed a small mistake, would be great if you could update it. 
I changed the word 'building' to 'build' in line 20. 
"Web application frameworks are made to help developers building web applications"
Also dup `column_types` in AR::Result `initialize_copy`
Array#in_groups: documentation updated
Fix missing line and shadow on static error pages [ci skip]
Merge pull request #12841 from kuldeepaggarwal/in-group-documentation

Array#in_groups: documentation updated [ci skip]
autoload ChangeColumnDefinition (Fixes #12835)
Speed up Array#split
Ruby 2.0.0p247
Rehearsal ---------------------------------------
old  10.670000   0.150000  10.820000 ( 10.822651)
new   8.520000   0.050000   8.570000 (  8.571825)
----------------------------- total: 19.390000sec

          user     system      total        real
old  10.620000   0.170000  10.790000 ( 10.790409)
new   8.570000   0.110000   8.680000 (  8.686051)
Merge pull request #12842 from kuldeepaggarwal/array_split

Speed up Array#split when block is passed
Merge pull request #12843 from DeltaKilo/css_fix

Fix missing line and shadow on static error pages [ci skip]
Merge pull request #12844 from akalyaev/bugfix/12835-autoload-change-column-definition

autoload ChangeColumnDefinition (Fixes #12835)
Refer to Rails.env only when Rails is defined
A tiny grammatical fix

[ci skip]
Fixes problem with replacing has_one association record with itself
Merge pull request #12834 from dredozubov/has_one_association_replacement

Fixes problem with replacing has_one association record with itself
Load test fixtures where data are needed

Without this, some tests here were not actually testing anything.
Avoid sorting an Array including objects from different Classes

addresses "ArgumentError: comparison of VerySpecialComment with SpecialComment failed" in ActiveRecord::DelegationRelationTest#test_#sort!_delegation_is_deprecated
tidy Active Record CHANGELOG. [ci skip].
Fixed typo in migrations guide

Product model name was pluralized in example in migrations guide.
[ci skip]
Merge pull request #12847 from jipe/master

Fixed typo in migrations guide [ci skip]