Tools/fsi.bat: add error handling

Otherwise we would get the following error when running
configure.bat in geewallet (after submodule population):

```
'""' is not recognized as an internal or external command,
operable program or batch file.
```

Tools/fsi.bat

@@ -1,5 +1,24 @@
 @ECHO OFF
 
-FOR /f "tokens=* delims=" %%A in ('"%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe" -latest -requires Microsoft.VisualStudio.Component.FSharp -find **\fsi.exe') do set RUNNER=%%A
+SET "VSWHERE=%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe"
+SET "INSTALL_MSG=Please install .NET v6, or higher; or Visual Studio."
+
+ECHO checking for '%VSWHERE%'...
+IF NOT EXIST "%VSWHERE%" (
+    echo:
+    echo Tool vswhere.exe not found.
+    echo %INSTALL_MSG%
+    exit /b 1
+)
+
+ECHO checking for fsi.exe...
+FOR /f "tokens=* delims=" %%A in ('"%VSWHERE%" -latest -requires Microsoft.VisualStudio.Component.FSharp -find **\fsi.exe') do set RUNNER=%%A
+
+IF "%RUNNER%"=="" (
+    echo:
+    echo F# not found.
+    echo %INSTALL_MSG%
+    exit /b 1
+)
 
 "%RUNNER%" --define:LEGACY_FRAMEWORK %*