throw custom error when state_type is invalid

spacy/errors.py

@@ -480,6 +480,8 @@ class Errors:
     E201 = ("Span index out of range.")
 
     # TODO: fix numbering after merging develop into master
+    E917 = ("Received invalid value {value} for 'state_type' in "
+            "TransitionBasedParser: only 'parser' or 'ner' are valid options.")
     E918 = ("Received invalid value for vocab: {vocab} ({vocab_type}). Valid "
             "values are an instance of spacy.vocab.Vocab or True to create one"
             " (default).")

spacy/ml/models/parser.py

@@ -2,6 +2,7 @@ from typing import Optional, List
 from thinc.api import Model, chain, list2array, Linear, zero_init, use_ops
 from thinc.types import Floats2d
 
+from ... import Errors
 from ...util import registry
 from .._precomputable_affine import PrecomputableAffine
 from ..tb_framework import TransitionModel
@@ -66,7 +67,7 @@ def build_tb_parser_model(
     elif state_type == "ner":
         nr_feature_tokens = 6 if extra_state_tokens else 3
     else:
-        raise ValueError(f"unknown state type {state_type}")  # TODO error
+        raise ValueError(Errors.E917.format(value=state_type))
     t2v_width = tok2vec.get_dim("nO") if tok2vec.has_dim("nO") else None
     tok2vec = chain(tok2vec, list2array(), Linear(hidden_width, t2v_width))
     tok2vec.set_dim("nO", hidden_width)