pvmfw: Panic on fill_with_entropy error for BSSL

Panic instead of silently ignoring the returned error and returning
potentially invalid (or incomplete) entropy to the BSSL caller of
CRYPTO_sysrand.

Test: TH
Bug: 288384378
Change-Id: I046f921608445586e899467eef31eb4699990a8f
diff --git a/pvmfw/src/rand.rs b/pvmfw/src/rand.rs
index bf0edd5..b45538a 100644
--- a/pvmfw/src/rand.rs
+++ b/pvmfw/src/rand.rs
@@ -42,6 +42,12 @@
     }
 }
 
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{self}")
+    }
+}
+
 /// Configure the source of entropy.
 pub fn init() -> Result<()> {
     match hvc::trng_version()? {
@@ -105,5 +111,5 @@
 extern "C" fn CRYPTO_sysrand(out: *mut u8, req: usize) {
     // SAFETY - We need to assume that out points to valid memory of size req.
     let s = unsafe { core::slice::from_raw_parts_mut(out, req) };
-    let _ = fill_with_entropy(s);
+    fill_with_entropy(s).unwrap()
 }