Various error fixes.

Fixed incorrect error types.  Added missing error string prefixes.
Removed some invalid or superfluous error emissions.  Clarified some
error strings.  Joined error string lines to aid grep.  Replaced some
generic error strings with specific ones.  Documentation work.

Fixes #450.

src/win32_monitor.c

@@ -144,7 +144,7 @@ _GLFWmonitor** _glfwPlatformGetMonitors(int* count)
             if (!name)
             {
                 _glfwInputError(GLFW_PLATFORM_ERROR,
-                                "Failed to convert string to UTF-8");
+                                "Win32: Failed to convert string to UTF-8");
                 continue;
             }