Add a more helpful error message when precompiling fails due to Python build change (#778)

* Add a more helpful error message when libpython is not found

* remove pointer to nothing

* Add additional if in case err does not have msg field

* Change tabs to spaces

* remove error message when object is not an errorException.

* remove loading library twice

Co-authored-by: Nisha Chandramoorthy <nishac92@gmail.com>

Author: Nisha Chandramoorthy

src/startup.jl

@@ -44,7 +44,15 @@ if !symbols_present
     isfile(depfile) || error("PyCall not properly installed. Please run Pkg.build(\"PyCall\")")
     include(depfile) # generated by Pkg.build("PyCall")
     # Only to be used at top-level - pointer will be invalid after reload
-    libpy_handle = Libdl.dlopen(libpython, Libdl.RTLD_LAZY|Libdl.RTLD_DEEPBIND|Libdl.RTLD_GLOBAL)
+    libpy_handle = try
+        Libdl.dlopen(libpython, Libdl.RTLD_LAZY|Libdl.RTLD_DEEPBIND|Libdl.RTLD_GLOBAL)
+    catch err
+        if err isa ErrorException
+            error(err.msg, ". Please run `Pkg.build(\"PyCall\")` if your Python build has changed")
+        else
+            rethrow(err)
+        end
+    end
     # need SetPythonHome to avoid warning, #299
     Py_SetPythonHome(libpy_handle, pyversion_build, PYTHONHOME)
 else