better parser error recovery for keyword as default argument value (PY-3713)

python/testData/psi/KeywordAsDefaultParameterValue.py

@@ -0,0 +1,2 @@
+def select2(self, fields=from):
+    pass

python/testData/psi/KeywordAsDefaultParameterValue.txt

@@ -0,0 +1,22 @@
+PyFile:KeywordAsDefaultParameterValue.py
+  PyFunction('select2')
+    PsiElement(Py:DEF_KEYWORD)('def')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('select2')
+    PyParameterList
+      PsiElement(Py:LPAR)('(')
+      PyNamedParameter('self')
+        PsiElement(Py:IDENTIFIER)('self')
+      PsiElement(Py:COMMA)(',')
+      PsiWhiteSpace(' ')
+      PyNamedParameter('fields')
+        PsiElement(Py:IDENTIFIER)('fields')
+        PsiElement(Py:EQ)('=')
+        PsiErrorElement:expression expected
+          PsiElement(Py:FROM_KEYWORD)('from')
+      PsiElement(Py:RPAR)(')')
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n    ')
+    PyStatementList
+      PyPassStatement
+        PsiElement(Py:PASS_KEYWORD)('pass')