Mount all partitions before running postinstall

For Virtual AB Compression, we need to mount snapshotted partitions
manually.

Test: treehugger
Change-Id: I15af8375e393ce2539d2075b467e8dbe94bec247
diff --git a/common/error_code_utils.cc b/common/error_code_utils.cc
index cda4c7e..421544a 100644
--- a/common/error_code_utils.cc
+++ b/common/error_code_utils.cc
@@ -173,6 +173,8 @@
       return "ErrorCode::kDeviceCorrupted";
     case ErrorCode::kPackageExcludedFromUpdate:
       return "ErrorCode::kPackageExcludedFromUpdate";
+    case ErrorCode::kPostInstallMountError:
+      return "ErrorCode::kPostInstallMountError";
       // Don't add a default case to let the compiler warn about newly added
       // error codes which should be added here.
   }