why MongoDB send error 500 when duplicated key - mongodb

Question

when recived the answer from MongoDB, know that my error is a duplicate key but why status=500 ?, it should be 4**.
I'm using nodejs (sails/express.js)
{ "error": {
"error": "E_UNKNOWN",
"status": 500,
"summary": "Encountered an unexpected error",
"raw": {
"name": "MongoError",
"code": 11000,
"err": "E11000 duplicate key error index: eReporterDB.users.$name_1 dup key: { : \"codin\" }"
} } }

Answer 1

the answer is here for nodejs
Operational errors vs. programmer errors
It's helpful to divide all errors into two broad categories:
Operational errors represent run-time problems experienced by correctly-written programs. These are not bugs in the program. In
fact, these are usually problems with something else: the system
itself (e.g., out of memory or too many open files), the system's
configuration (e.g., no route to a remote host), the network (e.g.,
socket hang-up), or a remote service (e.g., a 500 error, failure to
connect, or the like). Examples include:
failed to connect to server
failed to resolve hostname
**invalid user input**
request timeout
server returned a 500 response
socket hang-up
system is out of memory
Programmer errors are bugs in the program. These are things that can always be avoided by changing the code. They can never be handled
properly (since by definition the code in question is broken).
tried to read property of "undefined"
called an asynchronous function without a callback
passed a "string" where an object was expected
passed an object where an IP address string was expected