evalVariable: Thows ShellVariableError if no variable is found

src/fsh.nim

@@ -161,7 +161,11 @@ proc evalVariable(vari: string): string =
     result=shellVariables[vari]
   elif envVariables.hasKey(vari):
     result = envVariables[vari]
-  else: raise newException(Exception, "No such variable: " & vari & ".")
+  else:
+    var e = newException(ShellVariableError, "No such variable: " & vari)
+    e.variable = vari
+    e.msg = "No such variable: " & e.variable
+    raise e
 
 # substitute does a substitution on the stream passed in
 proc substitute(strm: Stream, delim=";\p"): string =