box API to return last error (code, message, type, errno, etc)

[rtsisyk]

Implement two new calls:
box.error.last() - returns last error, if any, as a Lua table
box.error.clear() - clears the last error
And add an alias for box.error():
box.error.raise()

Example:

box.error.last()

---
- null
...
box.error({code = 123, reason = 'test'})

---
- error: test
...
box.error(box.error.ILLEGAL_PARAMS, "bla bla")

---
- error: Illegal parameters, bla bla
...
box.error.last()

---
- type: ClientError
  message: Illegal parameters, bla bla
  code: 1
...
box.error.clear()

---
...
box.error.last()

---
- null
...

Observe, that the returned lua table has three members:

• type: C++ class type of the error
• message: string message for the error
• code: numeric code of the error, as passed to box.error(), usually one of the codes in box.error Lua table

Additionally, if the error is a system error (socket, file or memory related) a new member 'errno' contains C standard errno.