Replace most sys.exit() with raising exceptions

Because ruffus doesn’t handle exceptions well I tended to call sys.exit
to make sure we got out of dodge when needed.  However, sys.exit is not
ideal for the Python API this is moving towards, so this introduces
proper exceptions for the various cases that retain suggested error
codes. Only __main__.py should call sys.exit now, everyone else has to
throw an exception.

For now the worker raising a fatal exception is logging messages rather
than passing an exception object with the fatal error message, mainly
because ruffus doesn’t properly marshall the exception object so we
just check “what is the name of the exception class that caused ruffus
to thrown an RethrownJobError”?

Also fixed along the way was the wrong return code being shown for
encrypted PDF checking, and incorrect use of str.find (e.output.find)
in boolean logic (str.find returns -1 on failure to find, which is True).

tests/test_main.py

@@ -10,7 +10,7 @@ import sys
 import pytest
 from ocrmypdf.pageinfo import pdf_get_all_pageinfo
 import PyPDF2 as pypdf
-from ocrmypdf import ExitCode
+from ocrmypdf.exceptions import ExitCode
 from ocrmypdf import leptonica
 from ocrmypdf.pdfa import file_claims_pdfa
 import platform
@@ -61,11 +61,6 @@ def check_ocrmypdf(input_basename, output_basename, *args, env=None):
 
     p, out, err = run_ocrmypdf(input_basename, output_basename, *args, env=env)
     print(err)  # ensure py.test collects the output, use -s to view
-    if p.returncode != 0:
-        print('stdout\n======')
-        print(out)
-        print('stderr\n======')
-        print(err)
     assert p.returncode == 0
     assert os.path.exists(output_file), "Output file not created"
     assert os.stat(output_file).st_size > 100, "PDF too small or empty"
@@ -88,6 +83,8 @@ def run_ocrmypdf(input_basename, output_basename, *args, env=None):
         p_args, close_fds=True, stdout=PIPE, stderr=PIPE,
         universal_newlines=True, env=env)
     out, err = p.communicate()
+    print(err)
+
     return p, out, err
 
 
@@ -548,15 +545,16 @@ def test_qpdf_repair_fails():
     env['OCRMYPDF_QPDF'] = os.path.abspath('./spoof/qpdf_dummy_return2.py')
     p, out, err = run_ocrmypdf(
         '-v', '1',
-        'c02-22.pdf', 'wont_be_created.pdf', env=env)
+        'c02-22.pdf', 'wont_be_created_repair_fail.pdf', env=env)
     print(out)
     print(err)
     assert p.returncode == ExitCode.input_file
 
 
 def test_encrypted():
-    p, out, err = run_ocrmypdf('skew-encrypted.pdf', 'wont_be_created.pdf')
-    assert p.returncode == ExitCode.input_file
+    p, out, err = run_ocrmypdf(
+        'skew-encrypted.pdf', 'wont_be_created_test_enc.pdf')
+    assert p.returncode == ExitCode.encrypted_pdf
     assert out.find('password')