Fix crash in gdbpy_parse_register_id

I noticed that gdbpy_parse_register_id would assert if passed a Python
object of a type it was not expecting.  The included test case shows
this crash.  This patch fixes the problem and also changes
gdbpy_parse_register_id to be more "Python-like" -- it always ensures
the Python error is set when it fails, and the callers now simply
propagate the existing exception.

gdb/testsuite/gdb.python/py-frame.exp

@@ -134,3 +134,9 @@ gdb_test "python print(gdb.selected_frame().language())" "c"
 gdb_test "set language ada"
 gdb_test "python print(gdb.selected_frame().language())" "c" \
     "frame language is not affected by global language"
+
+# This previously caused a crash -- the implementation was missing the
+# case where a register had an unexpected type.
+gdb_test "python print(gdb.selected_frame().read_register(list()))" \
+    ".*Invalid type for register.*" \
+    "test Frame.read_register with list"

gdb/testsuite/gdb.python/py-unwind.exp

@@ -57,3 +57,9 @@ gdb_test_sequence "where"  "Backtrace restored by unwinder" {
 
 # Check that the Python unwinder frames can be flushed / released.
 gdb_test "maint flush register-cache" "Register cache flushed\\." "flush frames"
+
+# Check that invalid register names cause errors.
+gdb_test "python print(add_saved_register_error)" "True" \
+    "add_saved_register error"
+gdb_test "python print(read_register_error)" "True" \
+    "read_register error"

gdb/testsuite/gdb.python/py-unwind.py

@@ -17,6 +17,11 @@ import gdb
 from gdb.unwinder import Unwinder
 
 
+# These are set to test whether invalid register names cause an error.
+add_saved_register_error = False
+read_register_error = False
+
+
 class FrameId(object):
     def __init__(self, sp, pc):
         self._sp = sp
@@ -101,6 +106,12 @@ class TestUnwinder(Unwinder):
             previous_ip = self._read_word(bp + 8)
             previous_sp = bp + 16
 
+            try:
+                pending_frame.read_register("nosuchregister")
+            except ValueError:
+                global read_register_error
+                read_register_error = True
+
             frame_id = FrameId(
                 pending_frame.read_register(TestUnwinder.AMD64_RSP),
                 pending_frame.read_register(TestUnwinder.AMD64_RIP),
@@ -109,6 +120,11 @@ class TestUnwinder(Unwinder):
             unwind_info.add_saved_register(TestUnwinder.AMD64_RBP, previous_bp)
             unwind_info.add_saved_register("rip", previous_ip)
             unwind_info.add_saved_register("rsp", previous_sp)
+            try:
+                unwind_info.add_saved_register("nosuchregister", previous_sp)
+            except ValueError:
+                global add_saved_register_error
+                add_saved_register_error = True
             return unwind_info
         except (gdb.error, RuntimeError):
             return None