exception: create subclass for generated errors

All errors generated by sdbus++ YAML we want to follow the systemd
default errno value (EIO).  All exceptions defined otherwise it is
probably a good idea to ensure that the author picks an explicit
errno.

Create a base class for generated exceptions to hold the default
`get_errno() { return EIO; }` implementation, which can be used by
various repositories and later turn the exception_t one into a pure
virtual.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I2648825b677353970eed4724bd2d3710a103d814
diff --git a/src/exception.cpp b/src/exception.cpp
index 640454a..4fa5135 100644
--- a/src/exception.cpp
+++ b/src/exception.cpp
@@ -19,6 +19,11 @@
     return EIO;
 }
 
+int generated_exception::get_errno() const noexcept
+{
+    return EIO;
+}
+
 SdBusError::SdBusError(int error, const char* prefix, SdBusInterface* intf) :
     error(SD_BUS_ERROR_NULL), intf(intf)
 {