Document marina error_reason/0 and cql_error/0 in README

A2 standardization shipped both types as exports in 0.4.5, but the
README didn't enumerate them so callers had to peek at
include/marina.hrl to find the atoms. Added an 'Errors' subsection
under 'Usage' (alongside Query options) listing:

  - marina_pool_not_started, timeout
  - cql_error() = {pos_integer(), binary()} -- the server-side
    CQL error code + message tuple, linked to Cassandra's CQL
    error-codes documentation
  - no_server, pool_not_started, shackle_not_started

Plus a one-liner that cql_error/0 is publicly exported for clean
pattern-matching at call sites.

The 0.4.6 tag (the latest from this session, on top of 0.4.5) moves
forward to this commit so the hex publish includes the README
updates. 0.4.5's commit stays where it is -- the README polish
lands at the newer tag.

Author: lpgauth

README.md

@@ -115,6 +115,16 @@ Asynchronous (return a `shackle:request_id()`, consume via `marina:receive_respo
 | `timeout`           | `pos_integer()`                    | `1000`                    |
 | `values`            | `[binary()]`                       | `undefined`               |
 
+### Errors
+
+All `marina:*` calls return `{ok, term()} | {error, error_reason()}` where `error_reason/0` (exported, added in 0.4.5) enumerates:
+
+- **`marina_pool_not_started`**, **`timeout`** — marina-level routing / wait errors.
+- **`cql_error()`** — `{pos_integer(), binary()}`, the Cassandra/Scylla server-side error tuple. The integer is the [CQL error code](https://cassandra.apache.org/doc/latest/cassandra/developing/cql/error_codes.html); the binary is the server-supplied message.
+- **`no_server`**, **`pool_not_started`**, **`shackle_not_started`** — shackle-level errors that propagate through marina.
+
+`cql_error/0` is exported as a public type so callers can pattern-match against the server-error shape cleanly.
+
 ## Architecture notes
 
 - **Bootstrap.** Dial each `bootstrap_ip` in order, query `system.local` + `system.peers`, filter by the seed's datacenter, start one shackle pool per peer.