[ModelicaSystem.sendExpression] include (original) error message

'from ex' links the original exception but this is not always shown

see also OpenModelica#328

Author: syntron

OMPython/ModelicaSystem.py

@@ -537,7 +537,7 @@ def sendExpression(self, expr: str, parsed: bool = True) -> Any:
         try:
             retval = self._getconn.sendExpression(expr, parsed)
         except OMCSessionException as ex:
-            raise ModelicaSystemError(f"Error executing {repr(expr)}") from ex
+            raise ModelicaSystemError(f"Error executing {repr(expr)}: {ex}") from ex
 
         logger.debug(f"Result of executing {repr(expr)}: {textwrap.shorten(repr(retval), width=100)}")