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.

tests/sample_errors/empty.err

@@ -1 +1 @@
-1,0,0,File must start with a "using Gtk" directive (e.g. `using Gtk 4.0;`)
+1,1,0,File must start with a "using Gtk" directive (e.g. `using Gtk 4.0;`)

tests/sample_errors/expected_semicolon.err

@@ -1 +1 @@
-6,1,1,Expected `;`
+5,4,0,Expected `;`

tests/sample_errors/incomplete_signal.err

@@ -1,2 +1 @@
-5,1,0,Expected a signal detail name
-4,9,3,Unexpected tokens
+4,11,0,Expected a signal detail name

tests/sample_errors/menu_toplevel_attribute.err

@@ -1,2 +1 @@
-4,5,21,Attributes are not permitted at the top level of a menu
-4,16,10,Unexpected tokens
+4,5,21,Attributes are not permitted at the top level of a menu

tests/sample_errors/no_import_version.err

@@ -1 +1 @@
-1,11,0,Expected a version number for GTK
+1,10,0,Expected a version number for GTK

tests/test_samples.py

@@ -143,9 +143,9 @@ class TestSamples(unittest.TestCase):
             ]
 
             def error_str(error: CompileError):
-                line, col = utils.idx_to_pos(error.range.start + 1, blueprint)
+                line, col = utils.idx_to_pos(error.range.start, blueprint)
                 len = error.range.length
-                return ",".join([str(line + 1), str(col), str(len), error.message])
+                return ",".join([str(line + 1), str(col + 1), str(len), error.message])
 
             actual = "\n".join([error_str(error) for error in errors])