Python: Introduce gdb.RecordGap class

As discussed here: https://sourceware.org/ml/gdb-patches/2017-04/msg00157.html

A gap is not an instruction and it should not pretend to be one.
gdb.Record.instruction_history is now a list of gdb.RecordInstruction and
gdb.RecordGap objects.  This allows the user to deal with Gaps in the record
in a more sane way.

gdb/testsuite/gdb.python/py-record-btrace.exp

@@ -81,7 +81,6 @@ with_test_prefix "replay end" {
 
 with_test_prefix "instruction " {
     gdb_test "python print(i.number)" "1"
-    gdb_test "python print(i.error)" "None"
     gdb_test "python print(i.sal)" "symbol and line for .*"
     gdb_test "python print(i.pc)" "$decimal"
     if { $gdb_py_is_py3k == 0 } {