Fix #10755 - ICE: `--linker=`

Trap the io_error condition so that a more informative error message is
displayed when the linker program cannot be started, such as when the
name of the linker binary is accidentally mistyped.

closes #10755
auto merge of #10997 : cadencemarseille/rust/issue-10755-ICE-for-missing-linker, r=alexcrichton

Trap the io_error condition so that a more informative error message is
displayed when the linker program cannot be started, such as when the
name of the linker binary is accidentally mistyped.

closes #10755