Wrap UnexpectedCharacters error as ObservationConfigError

Wrap UnexpectedCharacters error in as ObservationConfigError containing the unexpected character, line# and its content

src/ert/parsing/new_observations_parser.py

@@ -2,7 +2,7 @@
 from enum import Enum, auto
 from typing import Any, Dict, List, Literal, Tuple, TypedDict, Union
 
-from lark import Lark, Transformer
+from lark import Lark, Transformer, UnexpectedCharacters
 from typing_extensions import NotRequired
 
 from .config_errors import ConfigValidationError
@@ -104,9 +104,30 @@ def _parse_content(
         Tuple[ObservationType, FileContextToken, Dict[FileContextToken, Any]],
     ]
 ]:
-    return (FileContextTransformer(filename) * TreeToObservations()).transform(
-        observations_parser.parse(content)
-    )
+    try:
+        return (FileContextTransformer(filename) * TreeToObservations()).transform(
+            observations_parser.parse(content)
+        )
+    except UnexpectedCharacters as e:
+        unexpected_char = e.char
+        allowed_chars = e.allowed
+        unexpected_line = content.splitlines()[e.line - 1]
+        message = (
+            f"Observation parsing failed: Did not expect character: {unexpected_char}"
+            f" (on line {e.line}: {unexpected_line}). "
+            f"Expected one of {allowed_chars}."
+        )
+
+        raise ObservationConfigError.from_info(
+            ErrorInfo(
+                filename=filename,
+                message=message,
+                line=e.line,
+                end_line=e.line + 1,
+                column=e.column,
+                end_column=e.column + 1,
+            )
+        )
 
 
 observations_parser = Lark(

tests/test_config_parsing/test_new_observations_parser.py

@@ -1,4 +1,10 @@
-from ert.parsing.new_observations_parser import ObservationType, _parse_content
+import pytest
+
+from ert.parsing.new_observations_parser import (
+    ObservationConfigError,
+    ObservationType,
+    _parse_content,
+)
 
 
 def test_parse():
@@ -68,3 +74,11 @@ def test_parse():
             ),
         ]
     )
+
+
+def test_that_unexpected_character_gives_observation_config_error():
+    with pytest.raises(
+        ObservationConfigError,
+        match=".*i.*line 1.*include a;",
+    ):
+        _parse_content(content="include a;", filename="")