fastboot driver: fix message

Test: pass
Bug: 173654501

Change-Id: I7d5e7ce817a5ec4e3aba6b44bab3149683a46fdd
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 5307a00..48d5521 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -1002,7 +1002,7 @@
 
     std::string partition_size_str;
     if (fb->GetVar("partition-size:" + partition, &partition_size_str) != fastboot::SUCCESS) {
-        die("cannot get boot partition size");
+        die("cannot get partition size for %s", partition.c_str());
     }
 
     partition_size_str = fb_fix_numeric_var(partition_size_str);