make FindRust gracefully fail if rustc not found

Earlier a missing .cargo/bin/rustc would fail with

The system cannot find the file specified
CMake Error at corrosion/cmake/FindRust.cmake:171 (get_filename_component):
  get_filename_component called with incorrect number of arguments

Now it prints a more descriptive errormessage, sets Rust_FOUND to false
and returns.
It also sets Rust_FOUND to true if a rust target is successfully created

Author: trondhe

cmake/FindRust.cmake

@@ -166,9 +166,19 @@ if (_RESOLVE_RUSTUP_TOOLCHAINS)
             NO_DEFAULT_PATH)
 else()
     find_program(Rust_COMPILER_CACHED rustc)
+    if (Rust_COMPILER_CACHED)
+        get_filename_component(_RUST_TOOLCHAIN_PATH ${Rust_COMPILER_CACHED} DIRECTORY)
+        get_filename_component(_RUST_TOOLCHAIN_PATH ${_RUST_TOOLCHAIN_PATH} DIRECTORY)
+    endif()
+endif()
 
-    get_filename_component(_RUST_TOOLCHAIN_PATH ${Rust_COMPILER_CACHED} DIRECTORY)
-    get_filename_component(_RUST_TOOLCHAIN_PATH ${_RUST_TOOLCHAIN_PATH} DIRECTORY)
+if (NOT Rust_COMPILER_CACHED)
+    message(
+        WARNING "The rustc executable was not found. "
+        "Rust not installed or ~/.cargo/bin not added to path? "
+        "Aborting further actions of find_package(Rust). ")
+    set(Rust_FOUND false)
+    return()
 endif()
 
 # Look for Cargo next to rustc.
@@ -358,4 +368,5 @@ if(NOT TARGET Rust::Rustc)
         TARGET Rust::Cargo
         PROPERTY IMPORTED_LOCATION ${Rust_CARGO_CACHED}
     )
+    set(Rust_FOUND true)
 endif()