Returning a string from a function call requires converting to string

Given the following ImageJ macro:

```
function pickFile(){
return File.openDialog("Pick a file");
}

function notBuggy(){
return toString(pickFile());
}

function buggy(){
return pickFile();
}

pickFile();
notBuggy();
buggy();
```

This will ask you to pick 3 files. The first 2 times work, but the 3rd one fails with:

```
Numeric return value expected in line 11.
return pickFile( <)> ;
```

FIJI on ImageJ 1.51p, Java 1.8.0, MacOS 10.12.5

Start by reproducing the macro from the issue in Fiji/ImageJ, focusing on the pickFile, notBuggy, and buggy function calls. Trace the macro interpreter's handling of function return values and compare the direct return with the toString-wrapped return. Done means buggy() returns the selected path without the numeric return value error.

Written by the indexing model from the issue text.