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()
  }
 }