Clean up HRESULT error handling

[timsneath]

We handle errors inconsistently here.

Win32 defines an HRESULT as a signed value:

typedef LONG HRESULT;
typedef long LONG;

Typically it is shown as a const value (e.g. 0x800401F0), but when assigned to a Dart int, that is of course a positive value. To minimize confusion and retain consistency with COM:

• An HRESULT is treated as a signed int in Dart, rather than a positive 0x80000000 value.
• COM error constants will be converted to signed 32-bit values

The primary change to the code is that a line like:

const CO_E_NOTINITIALIZED = 0x800401F0;

will become

final CO_E_NOTINITIALIZED = 0x800401F0.toSigned(32);

While this adds some pain to the constants.dart file, this will remove some .toUnsigned(32) calls in the code and therefore avoid the risk of a returned HRESULT failing an equality test with a constant.

We'll need to make some updates to the code to test this consistently; I suspect there's a bug farm lurking here already that this work will root out. Good opportunity for more tests.