Better handling of wsl nastyness

#1198 reports that cmd.exe
and powershell.exe might not be available in the WSL path. Try to use
absolute paths if relative ones do not work and provide better error
messages when this doesn't work.

autoload/firenvim.vim

@@ -108,7 +108,15 @@ function! s:get_windows_env_path(env) abort
                 if l:env[0] ==# '$'
                         let l:env = '%' . l:env[1:-1] . '%'
                 endif
-                let l:cmd_output = system(['cmd.exe', '/c', 'echo', l:env])
+                try
+                        let l:cmd_output = system(['cmd.exe', '/c', 'echo', l:env])
+                catch /E475:.*cmd.exe' is not executable/
+                        try
+                                let l:cmd_output = system(['/mnt/c/Windows/System32/cmd.exe', '/c', 'echo', l:env])
+                        catch /E475:.*cmd.exe' is not executable/
+                                throw 'Error: Firenvim could not find cmd.exe from WSL on your system. Please report this issue.'
+                        endtry
+                endtry
                 return cmd_output[match(l:cmd_output, 'C:\\'):-3]
         endif
         throw 'Used get_windows_env_path on non-windows platform!'
@@ -746,7 +754,29 @@ function! firenvim#install(...) abort
                         echo 'Creating registry key for ' . l:name . '. This may take a while. Script: ' . l:ps1_path
                         call s:maybe_execute('writefile', split(l:ps1_content, "\n"), l:ps1_path)
                         call s:maybe_execute('setfperm', l:ps1_path, 'rwx------')
-                        let o = s:maybe_execute('system', ['powershell.exe', '-Command', '-'], readfile(l:ps1_path))
+                        try
+                                let o = s:maybe_execute('system', ['powershell.exe', '-Command', '-'], readfile(l:ps1_path))
+                        catch /powershell.exe' is not executable/
+                                let l:failure = v:true
+                                let l:msg = 'Error: Firenvim could not find powershell.exe'
+                                " If the failure happened on wsl, try to use
+                                " an absolute path
+                                if s:is_wsl
+                                        let l:msg += ' from WSL'
+                                        try
+                                                let o = s:maybe_execute('system', ['/mnt/c/Windows/System32/WindowsPowerShell/v1.0/powershell.exe', '-Command', '-'], readfile(l:ps1_path))
+                                                let l:failure = v:false
+                                        catch /powershell.exe' is not executable/
+                                                let l:failure = v:true
+                                        endtry
+                                endif
+                                let l:msg += ' on your system. Please report this issue.'
+                                if l:failure
+                                        echomsg 'Note: created ' . l:ps1_path . " . You may try to run it manually by right-clicking from your file browser to complete Firenvim's installation."
+                                        throw l:msg
+                                endif
+                        endtry
+
                         if v:shell_error
                           echo o
                         endif