Fixed `assert.ErrorAs()` error log text

[albenik]

Summary

Improve message of assert.ErrorAs()

Changes

• Replace non-informative message like expected: %!q(**pgconn.PgError=0xc000114050) to the human readable one.
• Replaced simple output of "%s", err.Error() to "%T(%s)" for better readability.

Motivation

• First the %!q(**pgconn.PgError=0xc000114050) text is the broken format string
• Second — even fixed, resulting err.Error() provides not enough information to see what exactly type of error is expected in errors.As and what is provided.

Related issues

Closes #1115