Revert "Fix expression error string"

This reverts commit 46fc568.
The change was not backported to 3.40

docs/pyqgis_developer_cookbook/expressions.rst

@@ -96,7 +96,7 @@ The following example shows how to check if a given expression can be parsed cor
    exp = QgsExpression('1 + 1 = ')
    assert(exp.hasParserError())
 
-   assert(exp.parserErrorString() == '\nIncomplete expression. You might not have finished the full expression.')
+   assert(exp.parserErrorString() == '\nsyntax error, unexpected end of file')
 
 .. index:: Expressions; Evaluating