Bug 1664581 - Explicitly recommend putting `hg` in your `PATH` if `bootstrap` can't find it r=nalexander

Differential Revision: https://phabricator.services.mozilla.com/D90171

python/mozboot/bin/bootstrap.py

@@ -227,7 +227,17 @@ def clone(vcs, no_interactive):
         print('Mercurial is not installed. Mercurial is required to clone '
               'Firefox%s.' % (
                   ', even when cloning with Git' if vcs == 'git' else ''))
-        print('Try installing hg with `pip3 install Mercurial`.')
+        try:
+            # We're going to recommend people install the Mercurial package with
+            # pip3. That will work if `pip3` installs binaries to a location
+            # that's in the PATH, but it might not be. To help out, if we CAN
+            # import "mercurial" (in which case it's already been installed),
+            # offer that as a solution.
+            import mercurial  # noqa: F401
+            print('Hint: have you made sure that Mercurial is installed to a '
+                  'location in your PATH?')
+        except ImportError:
+            print('Try installing hg with `pip3 install Mercurial`.')
         return None
     if vcs == 'hg':
         binary = hg