api: Introduce header with errors and move existing into it

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Introduce BADCTSTATE error

This one is to be returned when container is in bad
state for an operation to proceed.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
api: Separete error for the case when RPC doesn't report one

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Rename

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Error communicating via RPC channel

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Errors for bad FS/NET type values

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Error code for bad option

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Add comment about "reserved" -1 value

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Error for invalid Net/FS argument

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Error for 'missing namespace' failure

Sometimes the namespace we need to proceed is not available.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Merge some error codes with similar meaning

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Error for 'not found' situation in _del request

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Error for FS-mounting issues

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>