Refine function names and type signatures (#886)

* First name consistency pass. More to come!

* Remove multitouch variable (hopefully this compiles!)

* Remove CreationError::NotSupported

* Add new error handling types

* Remove `get_` prefix from getters.

This is as per the Rust naming conventions recommended in
https://rust-lang-nursery.github.io/api-guidelines/naming.html#getter-names-follow-rust-convention-c-getter

* Make changes to Window position and size function signatures

* Remove CreationError in favor of OsError

* Begin updating iOS backend

* Change MonitorHandle::outer_position to just position

* Fix build on Windows and Linux

* Add Display and Error implementations to Error types

* Attempt to fix iOS build.

I can't actually check that this works since I can't cross-compile to
iOS on a Windows machine (thanks apple :/) but this should be one of
several commits to get it working.

* Attempt to fix iOS errors, and muck up Travis to make debugging easier

* More iOS fixins

* Add Debug and Display impls to OsError

* Fix Display impl

* Fix unused code warnings and travis

* Rename set_ime_spot to set_ime_position

* Add CHANGELOG entry

* Rename set_cursor to set_cursor_icon and MouseCursor to CursorIcon

* Organize Window functions into multiple, categorized impls

* Improve clarity of function ordering and docs in EventLoop

src/error.rs

@@ -0,0 +1,86 @@
+use std::fmt;
+use std::error;
+
+use platform_impl;
+
+/// An error whose cause it outside Winit's control.
+#[derive(Debug)]
+pub enum ExternalError {
+    /// The operation is not supported by the backend.
+    NotSupported(NotSupportedError),
+    /// The OS cannot perform the operation.
+    Os(OsError),
+}
+
+/// The error type for when the requested operation is not supported by the backend.
+#[derive(Clone)]
+pub struct NotSupportedError {
+    _marker: (),
+}
+
+/// The error type for when the OS cannot perform the requested operation.
+#[derive(Debug)]
+pub struct OsError {
+    line: u32,
+    file: &'static str,
+    error: platform_impl::OsError,
+}
+
+impl NotSupportedError {
+    #[inline]
+    #[allow(dead_code)]
+    pub(crate) fn new() -> NotSupportedError {
+        NotSupportedError {
+            _marker: ()
+        }
+    }
+}
+
+impl OsError {
+    #[allow(dead_code)]
+    pub(crate) fn new(line: u32, file: &'static str, error: platform_impl::OsError) -> OsError {
+        OsError {
+            line,
+            file,
+            error,
+        }
+    }
+}
+
+#[allow(unused_macros)]
+macro_rules! os_error {
+    ($error:expr) => {{
+        crate::error::OsError::new(line!(), file!(), $error)
+    }}
+}
+
+impl fmt::Display for OsError {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        formatter.pad(&format!("os error at {}:{}: {}", self.file, self.line, self.error))
+    }
+}
+
+impl fmt::Display for ExternalError {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        match self {
+            ExternalError::NotSupported(e) => e.fmt(formatter),
+            ExternalError::Os(e) => e.fmt(formatter),
+        }
+    }
+}
+
+impl fmt::Debug for NotSupportedError {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        formatter.debug_struct("NotSupportedError").finish()
+    }
+}
+
+impl fmt::Display for NotSupportedError {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        formatter.pad("the requested operation is not supported by Winit")
+    }
+}
+
+impl error::Error for OsError {}
+impl error::Error for ExternalError {}
+impl error::Error for NotSupportedError {}