FIX: Improve error message when files could not be fetched

As per #8 (comment)

templateflow/api.py

@@ -26,20 +26,37 @@ def get(template, **kwargs):
         template=template, return_type='file', **kwargs)]
 
     # Try plain URL fetch first
-    for filepath in [p for p in out_file
-                     if p.is_file() and p.stat().st_size == 0]:
+    s3_missing = [p for p in out_file
+                  if p.is_file() and p.stat().st_size == 0]
+    for filepath in s3_missing:
         _s3_get(filepath)
 
+    dl_missing = None
     if TF_USE_DATALAD:
-        for filepath in [p for p in out_file if not p.is_file()]:
+        dl_missing = [p for p in out_file if not p.is_file()]
+        for filepath in dl_missing:
             _datalad_get(filepath)
 
     not_fetched = [p for p in out_file
                    if not p.is_file() or p.stat().st_size == 0]
 
     if any(not_fetched):
-        raise RuntimeError(
-            "Could not fetch template files: %s" % ', '.join(not_fetched))
+        msg = "Could not fetch template files: %s." % ', '.join(not_fetched)
+        if dl_missing and not TF_USE_DATALAD:
+            msg += """\
+The $TEMPLATEFLOW_HOME folder %s seems to contain an initiated DataLad \
+dataset, but the environment variable $TEMPLATEFLOW_USE_DATALAD is not \
+set or set to one of (false, off, 0). Please set $TEMPLATEFLOW_USE_DATALAD \
+on (possible values: true, on, 1).""" % TF_LAYOUT.root
+
+        if s3_missing and TF_USE_DATALAD:
+            msg += """\
+The $TEMPLATEFLOW_HOME folder %s seems to contain an plain \
+dataset, but the environment variable $TEMPLATEFLOW_USE_DATALAD is \
+set to one of (true, on, 1). Please set $TEMPLATEFLOW_USE_DATALAD \
+off (possible values: false, off, 0).""" % TF_LAYOUT.root
+
+        raise RuntimeError(msg)
 
     if len(out_file) == 1:
         return out_file[0]