trivial: provide a little more context for disk size failures

While trying to build a new CI job, I came to realize that the
default image was too small, and the block-device-efi-lvm element
example wants 40GB by default.

Even though that can be easily resolved, it kind of requires taking
a look at the code to understand what is going on. So instead,
we'll now log an error as a hint.

Change-Id: I05bfeae1a2b9aef01f781115fdbd5046c80437f0

diskimage_builder/block_device/level1/partitioning.py

@@ -152,6 +152,9 @@ class Partitioning(PluginBase):
             # megabytes so it can align on sensible boundaries. And
             # create partitions right after previous so no need to
             # calculate start/end - just size.
+            if not size <= disk_free:
+                logger.error('The requested size of the image is smaller '
+                             'than the block device configuration.')
             assert size <= disk_free
             args['size'] = size // (1024 * 1024)