parser: Tweak parsing during error conditions

When an explicit parsing error is encountered and a CompileError raised,
apply the changes to the context state. This way, the rule that catches
the exception (e.g. Statement or Until) knows where the error occurred.

Also, changed "Expected" errors to be reported at the end of the
previous non-whitespace token.

blueprintcompiler/parse_tree.py

@@ -235,7 +235,15 @@ class ParseNode:
         start_idx = ctx.index
         inner_ctx = ctx.create_child()
 
-        if self._parse(inner_ctx):
+        try:
+            result = self._parse(inner_ctx)
+        except Exception as e:
+            # If an exception occurs, there's an explicit error, not just a rule that didn't match. Apply the context
+            # state so that whichever rule handles the exception (e.g. a Statement) knows where the error occurred.
+            ctx.apply_child(inner_ctx)
+            raise e
+
+        if result:
             ctx.apply_child(inner_ctx)
             if ctx.index == start_idx:
                 return ParseResult.EMPTY
@@ -269,11 +277,11 @@ class Err(ParseNode):
         if self.child.parse(ctx).failed():
             start_idx = ctx.start
             while ctx.tokens[start_idx].type in SKIP_TOKENS:
-                start_idx += 1
+                start_idx -= 1
             start_token = ctx.tokens[start_idx]
 
             raise CompileError(
-                self.message, Range(start_token.start, start_token.start, ctx.text)
+                self.message, Range(start_token.end, start_token.end, ctx.text)
             )
         return True
 
@@ -350,7 +358,20 @@ class Statement(ParseNode):
 
         token = ctx.peek_token()
         if str(token) != self.end:
-            ctx.errors.append(CompileError(f"Expected `{self.end}`", token.range))
+            start_idx = ctx.index - 1
+            while ctx.tokens[start_idx].type in SKIP_TOKENS:
+                start_idx -= 1
+            start_token = ctx.tokens[start_idx]
+
+            position = (
+                start_token.start if ctx.index - 1 == start_idx else start_token.end
+            )
+
+            ctx.errors.append(
+                CompileError(
+                    f"Expected `{self.end}`", Range(position, position, ctx.text)
+                )
+            )
         else:
             ctx.next_token()
         return True
@@ -411,7 +432,6 @@ class Until(ParseNode):
                         ctx.skip_unexpected_token()
             except CompileError as e:
                 ctx.errors.append(e)
-                ctx.next_token()
 
         return True