[otbn,dv] Improve error message on unknown encoding

This shouldn't ever happen, but a more helpful error message made
debugging things rather easier.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>

hw/ip/otbn/dv/uvm/env/otbn_env_cov.sv

diff --git a/hw/ip/otbn/dv/uvm/env/otbn_env_cov.sv b/hw/ip/otbn/dv/uvm/env/otbn_env_cov.sv
index 96ff69e..f0e0ba7 100644
--- a/hw/ip/otbn/dv/uvm/env/otbn_env_cov.sv
+++ b/hw/ip/otbn/dv/uvm/env/otbn_env_cov.sv
@@ -911,8 +911,8 @@
         enc_u_cg.sample(mnem, insn_data);
       "wcsr":
         enc_wcsr_cg.sample(mnem, insn_data);
-      default:
-        `DV_CHECK_FATAL(0, "Unknown encoding")
+      default: `dv_fatal($sformatf("Unknown encoding (%0s) for instruction `%0s'", encoding, mnem),
+                         `gfn)
     endcase
   endfunction