Fix error code on wrong snapshot ID or topology

When CreateVolumeRequest finds out that a volume to be created already
exists and it was restored from a *different* snapshot or is available at
different topology than requested, it should return ALREADY_EXISTS.

Author: jsafrane

Diff for: spec.md

@@ -1119,7 +1119,7 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 |-----------|-----------|-------------|-------------------|
 | Source incompatible or not supported | 3 INVALID_ARGUMENT | Besides the general cases, this code MUST also be used to indicate when plugin supporting CREATE_DELETE_VOLUME cannot create a volume from the requested source (`SnapshotSource` or `VolumeSource`). Failure MAY be caused by not supporting the source (CO SHOULD NOT have provided that source) or incompatibility between `parameters` from the source and the ones requested for the new volume. More human-readable information SHOULD be provided in the gRPC `status.message` field if the problem is the source. | On source related issues, caller MUST use different parameters, a different source, or no source at all. |
 | Source does not exist | 5 NOT_FOUND | Indicates that the specified source does not exist. | Caller MUST verify that the `volume_content_source` is correct, the source is accessible, and has not been deleted before retrying with exponential back off. |
-| Volume already exists but is incompatible | 6 ALREADY_EXISTS | Indicates that a volume corresponding to the specified volume `name` already exists but is incompatible with the specified `capacity_range`, `volume_capabilities` or `parameters`. | Caller MUST fix the arguments or use a different `name` before retrying. |
+| Volume already exists but is incompatible | 6 ALREADY_EXISTS | Indicates that a volume corresponding to the specified volume `name` already exists but is incompatible with the specified `capacity_range`, `volume_capabilities`, `parameters`, `accessibility_requirements` or `volume_content_source`. | Caller MUST fix the arguments or use a different `name` before retrying. |
 | Unable to provision in `accessible_topology` | 8 RESOURCE_EXHAUSTED | Indicates that although the `accessible_topology` field is valid, a new volume can not be provisioned with the specified topology constraints. More human-readable information MAY be provided in the gRPC `status.message` field. | Caller MUST ensure that whatever is preventing volumes from being provisioned in the specified location (e.g. quota issues) is addressed before retrying with exponential backoff. |
 | Unsupported `capacity_range` | 11 OUT_OF_RANGE | Indicates that the capacity range is not allowed by the Plugin, for example when trying to create a volume smaller than the source snapshot. More human-readable information MAY be provided in the gRPC `status.message` field. | Caller MUST fix the capacity range before retrying. |