Add context accessor functions for error status and string

[bjosv]

Add accessor functions to get the error status and string from contexts.
These functions replaces any direct access to err and errstr, and is a preparation for making contexts opaque.

Proposed naming:

/* Blocking API context */
int valkeyGetError(const valkeyContext *c);
const char *valkeyGetErrorString(const valkeyContext *c);

/* Async API context */
int valkeyAsyncGetError(const valkeyAsyncContext *ac);
const char *valkeyAsyncGetErrorString(const valkeyAsyncContext *ac);

/* Blocking Cluster API context */
int valkeyClusterGetError(const valkeyClusterContext *cc);
const char *valkeyClusterGetErrorString(const valkeyClusterContext *cc);

/* Async Cluster API context */
int valkeyClusterAsyncGetError(const valkeyClusterAsyncContext *acc);
const char *valkeyClusterAsyncGetErrorString(const valkeyClusterAsyncContext *acc);

Examples and tests have been updated, but READMEs needs to be updated too.

[bjosv]

The following naming variant has also been mentioned in issues/discussions:

int valkeyContextGetErr(const valkeyContext *c);
const char *valkeyContextGetErrStr(const valkeyContext *c);

[michael-grunder]

Works for me. I think the naming is good too. Probably not worth saving a few characters going to Err.