add custom warning when run_command fails

spacy/errors.py

@@ -539,6 +539,7 @@ class Errors(object):
     E199 = ("Unable to merge 0-length span at doc[{start}:{end}].")
 
     # TODO: fix numbering after merging develop into master
+    E970 = ("Can not execute command '{str_command}'. Do you have '{tool}' installed?")
     E971 = ("Found incompatible lengths in Doc.from_array: {array_length} for the "
             "array and {doc_length} for the Doc itself.")
     E972 = ("Example.__init__ got None for '{arg}'. Requires Doc.")

spacy/util.py

@@ -452,7 +452,12 @@ def run_command(command: Union[str, List[str]]) -> None:
     """
     if isinstance(command, str):
         command = split_command(command)
-    status = subprocess.call(command, env=os.environ.copy())
+    try:
+        status = subprocess.call(command, env=os.environ.copy())
+    except FileNotFoundError:
+        raise FileNotFoundError(
+            Errors.E970.format(str_command=" ".join(command), tool=command[0])
+        )
     if status != 0:
         sys.exit(status)