Update error message for custom-function validation

[xterao]

When the doma.expr.functions property cannot be retrieved from doma.compile.config, change the error text as follows:

Before:

An undefined built-in or custom function [{errorMethodName}] is being called in class []

After:

Custom function [{errorMethodName}] not found in class [ {configuredClassName}]

※{configuredClassName}=org.seasar.doma.expr.ExpressionFunctions

Here, {configuredClassName} is the class name specified in the doma.expr.functions property; if that property is unset, it defaults to org.seasar.doma.expr.ExpressionFunctions.