version-handler: Produce errors for unavailable data

We want to make it clear to the reader that the data is actually
unavailable vs reading outside the bounds of the data space.

Change-Id: Ifb63dfe600c268af9e7df24397c28e20316dbb8f
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/bmc/version-handler/version_handler.cpp b/bmc/version-handler/version_handler.cpp
index 0b77f80..7cccf62 100644
--- a/bmc/version-handler/version_handler.cpp
+++ b/bmc/version-handler/version_handler.cpp
@@ -118,7 +118,11 @@
                                               uint32_t requestedSize)
 {
     auto& data = sessionInfoMap.at(session)->data;
-    if (data == nullptr || !*data || (*data)->size() < offset)
+    if (data == nullptr || !*data)
+    {
+        throw std::runtime_error("Version data not ready for read");
+    }
+    if ((*data)->size() < offset)
     {
         return {};
     }