Show error when trying to run in folder that doesn't exist instead of assertion (#23242)

Closes #23240

Fixes regression caused by #23017

compiler/main.nim

@@ -326,7 +326,8 @@ proc mainCommand*(graph: ModuleGraph) =
     # so by default should not end up in $PWD nor in $projectPath.
     var ret = if optUseNimcache in conf.globalOptions: getNimcacheDir(conf)
               else: conf.projectPath
-    doAssert ret.string.isAbsolute # `AbsoluteDir` is not a real guarantee
+    if not ret.string.isAbsolute: # `AbsoluteDir` is not a real guarantee
+      rawMessage(conf, errCannotOpenFile, ret.string & "/")
     if conf.cmd in cmdDocLike + {cmdRst2html, cmdRst2tex, cmdMd2html, cmdMd2tex}:
       ret = ret / htmldocsDir
     conf.outDir = ret

tests/misc/t23240.nim

@@ -0,0 +1,6 @@
+discard """
+  cmd: "nim c foo/bar.nim"
+  action: "reject"
+  errormsg: "cannot open 'foo/'"
+  file: ""
+"""