fix(base-node/grpc): fixes panic if invalid kernel mr is given (#4693)

Description
---
Fixes panic if invalid kernel mr is given via grpc

Motivation and Context
---
Panic can be triggered by user-controller input 

How Has This Been Tested?
---
Not tested

applications/tari_app_grpc/src/conversions/block_header.rs

@@ -85,7 +85,7 @@ impl TryFrom<grpc::BlockHeader> for BlockHeader {
             output_mr: FixedHash::try_from(header.output_mr).map_err(|err| err.to_string())?,
             witness_mr: FixedHash::try_from(header.witness_mr).map_err(|err| err.to_string())?,
             output_mmr_size: header.output_mmr_size,
-            kernel_mr: FixedHash::try_from(header.kernel_mr).expect("Array size 32 cannot fail"),
+            kernel_mr: FixedHash::try_from(header.kernel_mr).map_err(|err| err.to_string())?,
             kernel_mmr_size: header.kernel_mmr_size,
             total_kernel_offset,
             total_script_offset,