Validate IO in SortBam to provide nicer exceptions (#994)

Using the `Io.assert*` within the constructor will yield better error messages for the user.

Also added an explicit close to the input `SamSource`

src/main/scala/com/fulcrumgenomics/bam/SortBam.scala

@@ -63,13 +63,15 @@ class SortBam
   @arg(flag='s', doc="Order into which to sort the records.") val sortOrder: SamOrder = SamOrder.Coordinate,
   @arg(flag='m', doc="Max records in RAM.") val maxRecordsInRam: Int = SamWriter.DefaultMaxRecordsInRam
 ) extends FgBioTool with LazyLogging {
+
+  Io.assertReadable(input)
+  Io.assertCanWriteFile(output)
+
   override def execute(): Unit = {
-    Io.assertReadable(input)
-    Io.assertCanWriteFile(output)
-
     val in  = SamSource(input)
     val out = SamWriter(output, in.header.clone(), sort=Some(sortOrder), maxRecordsInRam=maxRecordsInRam)
     out ++= in
     out.close()
+    in.safelyClose()
   }
 }