add $postgres_error variable

[niquola]

No description provided.

[0x6e6562]

Could you give an example of how you would use this?

[niquola]

This patch is implementation of ideas discussed in #8.
$postgres_error variable could be used by other modules to report error:

add_after_body $postgres_error;
more_set_headers "X-Error: $postgres_error";

In our case, we've created simple postgresql web console based on ngx_postgres and showing error messages is critical for UX.