Remove RuntimeError / CompilationError. (#2258)

Instead, work out the prefix for an error based on whether it was produced during parsing, semantic analysis, or when running the program.

explorer/interpreter/action_stack.cpp

@@ -91,7 +91,7 @@ auto ActionStack::ValueOfNode(ValueNodeView value_node,
     }
   }
   // TODO: Move these errors to compile time and explain them more clearly.
-  return RuntimeError(source_loc)
+  return ProgramError(source_loc)
          << "could not find `" << value_node.base() << "`";
 }