Add Windows install advice

src/ocrmypdf/_validation.py

@@ -446,7 +446,7 @@ def report_output_file_size(options, input_file, output_file):
 def check_dependency_versions(options):
     check_external_program(
         program='tesseract',
-        package={'darwin': 'tesseract', 'linux': 'tesseract-ocr'},
+        package={'linux': 'tesseract-ocr'},
         version_checker=tesseract.version,
         need_version='4.0.0',  # using backport for Travis CI
     )

src/ocrmypdf/exec/__init__.py

@@ -132,23 +132,33 @@ On RPM-based systems (Red Hat, Fedora), search for instructions on
 installing the RPM for {program}.
 '''
 
+windows_install_advice = '''
+If not already installed, install the Chocolatey package manager. Then use
+a command prompt to install the missing package:
+    choco install {package}
+'''
+
 
 def _get_platform():
     if sys.platform.startswith('freebsd'):
         return 'freebsd'
     elif sys.platform.startswith('linux'):
         return 'linux'
+    elif sys.platform.startswith('win'):
+        return 'windows'
     return sys.platform
 
 
 def _error_trailer(program, package, **kwargs):
     if isinstance(package, Mapping):
-        package = package[_get_platform()]
+        package = package.get(_get_platform(), program)
 
     if _get_platform() == 'darwin':
         log.info(osx_install_advice.format(**locals()))
     elif _get_platform() == 'linux':
         log.info(linux_install_advice.format(**locals()))
+    elif _get_platform() == 'windows':
+        log.info(windows_install_advice.format(**locals()))
 
 
 def _error_missing_program(program, package, required_for, recommended):