Expand '~' in :tangle switch; Improve error if dest dir absent

Fixes #13.

Author: kaushalmodi

src/ntangle.nim

@@ -157,7 +157,7 @@ proc parseTangleHeaderProperties(file: string, lnum: int, haObj: LangAndArgs) =
       of "no":
         bufEnabled = false
       else:               #filename
-        outfile = argval
+        outfile = argval.expandTilde
         if (not outfile.startsWith "/"): # if relative path
           outfile = dir / outfile
     of "padline":
@@ -474,11 +474,12 @@ proc writeFiles() =
       dataUpdated = data
     dbg "  outDir: `{outDir}'"
     if outDir != "":
-      if (not dirExists(outDir)) and fileHeaderArgs[file].mkdirp:
-        echo fmt"  Creating {outDir}/ .."
-        createDir(outDir)
       if (not dirExists(outDir)):
-        raise newException(IOError, fmt"Unable to write to `{file}'. `{outDir}/' directory does not exist.")
+        if fileHeaderArgs[file].mkdirp:
+          echo fmt"  Creating {outDir}/ .."
+          createDir(outDir)
+        else:
+          raise newException(UserError, fmt"Unable to write to `{file}' as `{outDir}/' directory does not exist. Set ':mkdirp yes' header arg to auto-create it.")
 
     if fileHeaderArgs[file].shebang != "":
       dataUpdated = fileHeaderArgs[file].shebang & "\n" & data

tests/mkdirp_no/mkdirp_no.org

@@ -1,3 +1,3 @@
-#+begin_src shell :tangle "./foo/bar/mkdirp_test.sh"
+#+begin_src shell :tangle "~/foo/bar/mkdirp_test.sh"
 echo "Hello"
 #+end_src

tests/test.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# Time-stamp: <2018-10-07 23:27:20 kmodi>
+# Time-stamp: <2018-10-08 16:54:50 kmodi>
 
 set -euo pipefail # http://redsymbol.net/articles/unofficial-bash-strict-mode
 IFS=$'\n\t'
@@ -21,8 +21,11 @@ run_test () {
     "${bin}" tests/missing_arg_value/missing_arg_value.org || true
     "${bin}" tests/invalid_arg_no_colon/ || true
 
-    rm -rf ./tests/foo/bar/
-    "${bin}" tests/mkdirp_no/mkdirp_no.org || true
+    TEMP_HOME="${repo_root}/tests/mkdirp_no"
+    rm -rf "${TEMP_HOME}/foo/"
+    HOME="${TEMP_HOME}" "${bin}" tests/mkdirp_no/mkdirp_no.org || true
+
+    rm -rf "${repo_root}/tests/mkdirp_yes/foo/"
     "${bin}" tests/mkdirp_yes/mkdirp_yes.org
 
     # Test tangling an Org file in the same dir.