Check file exists in {.compile.} pragma (#21105)

* Add test

* Check file exists before adding it into compilation

* Make error message look like other error messages

i.e. following the format `error msg: file`

tests/pragmas/tcompile_missing_file.nim

@@ -0,0 +1,5 @@
+discard """
+  joinable: false
+  errormsg: "cannot find: noexist.c"
+"""
+{.compile: "noexist.c".}