Issue a better error message if the target is unsupported. (#110)

Following an idea from in #76, add a sys.rs file, which only gets used
if the target isn't recognized, and use `compile_error` to give a
meaningful error message instead of cryptically failing on the compiler
being unable to open sys.rs.

For example, using --target=wasm32-unknown-unknown now gets this error:

```console
$ cargo check --target=wasm32-unknown-unknown
    Checking errno v0.3.11
error: The target OS is "unknown" or "none", so it's unsupported by the errno crate.
 --> src/sys.rs:8:1
  |
8 | compile_error!("The target OS is \"unknown\" or \"none\", so it's unsupported by the errno crate.");
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: could not compile `errno` (lib) due to 1 previous error
```

Author: sunfishcode

src/sys.rs

@@ -0,0 +1,35 @@
+//! A default sys.rs for unrecognized targets.
+//!
+//! If lib.rs doesn't recognize the target, it defaults to using this file,
+//! which issues an explanatory compile error.
+
+// If there is no OS, there's no `errno` or equivalent defined.
+#[cfg(any(target_os = "unknown", target_os = "none"))]
+compile_error!("The target OS is \"unknown\" or \"none\", so it's unsupported by the errno crate.");
+
+// If there is an OS, support may be added.
+#[cfg(not(any(target_os = "unknown", target_os = "none")))]
+compile_error!("The target OS is not yet supported in the errno crate.");
+
+// The following define the functions of the normal implementations
+// so that the user doesn't see uninteresting errors after the
+// errors above.
+
+use crate::Errno;
+
+pub fn with_description<F, T>(_err: Errno, _callback: F) -> T
+where
+    F: FnOnce(Result<&str, Errno>) -> T,
+{
+    unreachable!()
+}
+
+pub const STRERROR_NAME: &str = "";
+
+pub fn errno() -> Errno {
+    unreachable!()
+}
+
+pub fn set_errno(_: Errno) {
+    unreachable!()
+}