qrreader.get_qr_reader: raise instead of returning None

move MissingQrDetectionLib to core lib

electrum/qrreader/__init__.py

@@ -35,15 +35,17 @@ from .abstract_base import AbstractQrCodeReader, QrCodeResult
 _logger = get_logger(__name__)
 
 
-class MissingLib(RuntimeError):
-    ''' Raised by underlying implementation if missing libs '''
-    pass
+class MissingQrDetectionLib(RuntimeError):
+    ''' Raised if we can't find zbar or whatever other platform lib
+    we require to detect QR in image frames. '''
 
 
-def get_qr_reader() -> Optional[AbstractQrCodeReader]:
+def get_qr_reader() -> AbstractQrCodeReader:
     """
-    Get the Qr code reader for the current platform
+    Get the Qr code reader for the current platform.
+    Might raise exception: MissingQrDetectionLib.
     """
+    excs = []
     try:
         from .zbar import ZbarQrCodeReader
         return ZbarQrCodeReader()
@@ -59,5 +61,13 @@ def get_qr_reader() -> Optional[AbstractQrCodeReader]:
         """
     except MissingLib as e:
         _logger.exception("")
+        excs.append(e)
 
-    return None
+    raise MissingQrDetectionLib(f"The platform QR detection library is not available.\nerrors: {excs!r}")
+
+
+# --- Internals below (not part of external API)
+
+class MissingLib(RuntimeError):
+    ''' Raised by underlying implementation if missing libs '''
+    pass