[query] improve error message when rng_nonce is unparseable (hail-is#…

…12688)

* [query] improve error message when rng_nonce is unparseable

* Update ExecuteContext.scala

hail/src/main/scala/is/hail/backend/ExecuteContext.scala

@@ -5,7 +5,7 @@ import is.hail.{HailContext, HailFeatureFlags}
 import is.hail.annotations.{Region, RegionPool}
 import is.hail.expr.ir.Threefry
 import is.hail.io.fs.FS
-import is.hail.utils.{ExecutionTimer, using}
+import is.hail.utils._
 import is.hail.variant.ReferenceGenome
 
 import java.io._
@@ -111,7 +111,12 @@ class ExecuteContext(
 ) extends Closeable {
   var backendContext: BackendContext = _
 
-  val rngNonce: Long = java.lang.Long.decode(getFlag("rng_nonce"))
+  val rngNonce: Long = try {
+    java.lang.Long.decode(getFlag("rng_nonce"))
+  } catch {
+    case exc: NumberFormatException =>
+      fatal(s"Could not parse flag rng_nonce as a 64-bit signed integer: ${getFlag("rng_nonce")}", exc)
+  }
 
   private val tempFileManager: TempFileManager = if (_tempFileManager != null)
     _tempFileManager