Error when compiling for WASM (and other unsupported targets)

[kaimast]

Motivation

A project I work on (snarkOS) uses this crate (through self_update). We have certain features that do not compile on WebAssembly. There's unfortunately not a nice way to set up these conflicts in Cargo or build.rs.
When they build those features, they get a fairly useless error that does not indicate anything related to wasm.

So, I thought it would be best to add a check in this crate, as it only seems to support unix and windows. I also found a few typos in lib.rs that the PR fixes.

Outcome

Before, you get a fairly cryptic error:

error[E0308]: mismatched types
   --> src/lib.rs:148:50
    |
148 | pub fn self_delete_at<P: AsRef<Path>>(exe: P) -> Result<(), io::Error> {
    |        --------------                            ^^^^^^^^^^^^^^^^^^^^^ expected `Result<(), Error>`, found `()`
    |        |
    |        implicitly returns `()` as its body has no tail or `return` expression
    |
    = note:   expected enum `std::result::Result<(), std::io::Error>`
            found unit type `()`

error[E0308]: mismatched types
   --> src/lib.rs:165:58
    |
165 | pub fn self_delete_outside_path<P: AsRef<Path>>(p: P) -> Result<(), io::Error> {
    |        ------------------------                          ^^^^^^^^^^^^^^^^^^^^^ expected `Result<(), Error>`, found `()`
    |        |
    |        implicitly returns `()` as its body has no tail or `return` expression
    |
    = note:   expected enum `std::result::Result<(), std::io::Error>`
            found unit type `()`

error[E0308]: mismatched types
   --> src/lib.rs:199:59
    |
199 | pub fn self_replace<P: AsRef<Path>>(new_executable: P) -> Result<(), io::Error> {
    |        ------------                                       ^^^^^^^^^^^^^^^^^^^^^ expected `Result<(), Error>`, found `()`
    |        |
    |        implicitly returns `()` as its body has no tail or `return` expression
    |
    = note:   expected enum `std::result::Result<(), std::io::Error>`
            found unit type `()`

With this change, it simply prints:

error: self-replace cannot be built for this target (only windows and unix is supported)
   --> src/lib.rs:124:1
    |
124 | compile_error!("self-replace cannot be built for this target (only windows and unix is supported)");
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^