OvmfPkg/BaseMemEncryptSevLib: promote DEBUG_WARN levels to DEBUG_ERROR
In SetMemoryEncDec(), we have four locations where we (a) log a message on
the DEBUG_WARN level that says "ERROR", (b) return the status code
RETURN_NO_MAPPING right after.
These messages clearly describe actual errors (bad PML4, PDPE, PDE, PTE).
Promote their debug levels to DEBUG_ERROR, and remove the word "ERROR"
from the messages.