common, mount: Refactor mfs- and errno-style error handling

This commit unifies the functions for printing/converting error codes between
the ones used in LizardFS and these in errno.h.

Change-Id: I94ac2ff2fb11bfe31909fb4bcfcce88e0faf9f5a
mount: Change I/O limiting errors from errno-style to MFS status

This commit changes errors returned from I/O limiting from errno-style to MFS
status
to unify their use.

Change-Id: Idc1dd483ab2dfbc33e50b85dc65484ff2480a73e