问在rails迁移过程中捕获SQL错误-以便以静默方式失败

EN

class EnableExtensionRdkit < ActiveRecord::Migration
  def up
    begin
      ActiveRecord::Base.connection.execute("CREATE EXTENSION rdkit;")
    rescue => error
      p "NO RDKit SUPPORT due to exception: "+error.to_s
  end
  def down
    ActiveRecord::Base.connection.execute("DROP EXTENSION IF EXISTS rdkit;")
  end
end

$ rake db:migrate
==  EnableExtensionRdkit: migrating ===========================================
NO RDKit SUPPORT due to exception: PG::UndefinedFile: ERROR:  could not access file "$libdir/rdkit": No such file or directory
: CREATE EXTENSION rdkit;
==  EnableExtensionRdkit: migrated (0.0955s) ==================================

rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::InFailedSqlTransaction: ERROR:  current transaction is aborted, commands ignored until end of transaction block
:             SELECT attr.attname
            FROM pg_attribute attr
            INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[1]
            WHERE cons.contype = 'p'
              AND cons.conrelid = '"schema_migrations"'::regclass
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.1/lib/active_record/connection_adapters/postgresql_adapter.rb:774:in `async_exec'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.1/lib/active_record/connection_adapters/postgresql_adapter.rb:774:in `exec_no_cache'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.1/lib/active_record/connection_adapters/postgresql/database_statements.rb:138:in `block in exec_query'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.1/lib/active_record/connection_adapters/abstract_adapter.rb:435:in `block in log'
/Library/Ruby/Gems/2.0.0/gems/activesupport-4.0.1/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.1/lib/active_record/connection_adapters/abstract_adapter.rb:430:in `log'