Make error messages adhere to our standards

Make error messages adhere to our standards

* src/cygw32.c (chdir_to_default_directory):
* src/fns.c (secure_hash):
* src/keyboard.c (Finternal_handle_focus_in):
* src/keymap.c (store_in_keymap):
* src/pgtkfns.c (pgtk_set_scroll_bar_foreground)
(pgtk_set_scroll_bar_background, Fx_export_frames)
(Fpgtk_set_monitor_scale_factor, pgtk_get_defaults_value)
(pgtk_set_defaults_value, Fpgtk_print_frames_dialog)
(pgtk_get_monitor_scale_factor):
* src/pgtkterm.c (pgtk_set_parent_frame):
* src/process.c (network_interface_info, send_process):
* src/w32.c (w32_read_registry):
* src/w32fns.c (Fw32_read_registry):
* src/window.c (Frecenter):
* src/xfns.c (Fx_export_frames, Fx_print_frames_dialog)
(x_set_mouse_color): Make 'error' message strings follow our guidelines.
More specifically, they should not end in a period, and normally also be
capitalized.  See '(elisp) Programming Tips'.

(cherry picked from commit 970409916e0dff9cd4d542f16f7d570149bdeeb1)