Return useful error message when prepared statements fail to load.

Apps using sqerl get a silent failure to start message without any
useful information when the prepared statement source is broken.

This adds a error message with possibly useful information on the
source of the error.

For example, a mysterious error from pooler 'tried to add 5 members, only added 0' now also reports:

{incorrect_application_config,sqerl,
{prepared_statements,undef,
[{oc_chef_sql,statements,[pgsql],[]},
{sqerl_client,init,1,[{file,"src/sqerl_client.erl"},{line,126}]},
{gen_server,init_it,6,[{file,"gen_server.erl"},{line,304}]},
{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,227}]}]}}