fix: Report clear error if an input track does not exist (#94)

Probing with 'stream=index' should return the track_num if it existed, and an empty string if the input did not exist.

Fixes #89

streamer/autodetect.py

@@ -80,6 +80,13 @@ def _probe(input: Input, field: str) -> Optional[str]:
 
   return output_string
 
+def is_present(input: Input) -> bool:
+  """Returns true if the stream for this input is indeed found.
+
+  If we can't probe this input type, assume it is present."""
+
+  return bool(_probe(input, 'stream=index') or
+              input.input_type in TYPES_WE_CANT_PROBE)
 
 def get_language(input: Input) -> Optional[str]:
   """Returns the autodetected the language of the input."""

streamer/input_configuration.py

@@ -21,6 +21,21 @@
 from typing import List, Dict, Any, Optional
 
 
+class InputNotFound(configuration.ConfigError):
+  """An error raised when an input stream is not found."""
+
+  def __init__(self, input):
+    super().__init__(input.__class__, 'track_num',
+                     getattr(input.__class__, 'track_num'))
+    self.input = input
+
+  def __str__(self):
+    return ('In {}, {} track #{} was'
+            ' not found in "{}"').format(self.class_name,
+                                         self.input.media_type.value,
+                                         self.input.track_num,
+                                         self.input.name)
+
 class InputType(enum.Enum):
   FILE = 'file'
   """A track from a file.  Usable only with VOD."""
@@ -195,6 +210,9 @@ def __init__(self, *args) -> None:
     # modules.
     from . import autodetect
 
+    if not autodetect.is_present(self):
+      raise InputNotFound(self)
+
     def require_field(name: str) -> None:
       """Raise MissingRequiredField if the named field is still missing."""
       if getattr(self, name) is None: