[commands] Initial support for FlagConverter

The name is currently pending and there's no command.signature hook
for it yet since this requires bikeshedding.

discord/ext/commands/errors.py

@@ -75,6 +75,10 @@ __all__ = (
     'ExtensionFailed',
     'ExtensionNotFound',
     'CommandRegistrationError',
+    'BadFlagArgument',
+    'MissingFlagArgument',
+    'TooManyFlags',
+    'MissingRequiredFlag',
 )
 
 class CommandError(DiscordException):
@@ -855,3 +859,76 @@ class CommandRegistrationError(ClientException):
         self.alias_conflict = alias_conflict
         type_ = 'alias' if alias_conflict else 'command'
         super().__init__(f'The {type_} {name} is already an existing command or alias.')
+
+class FlagError(BadArgument):
+    """The base exception type for all flag parsing related errors.
+
+    This inherits from :exc:`BadArgument`.
+
+    .. versionadded:: 2.0
+    """
+    pass
+
+class TooManyFlags(FlagError):
+    """An exception raised when a flag has received too many values.
+
+    This inherits from :exc:`FlagError`.
+
+    .. versionadded:: 2.0
+
+    Attributes
+    ------------
+    flag: :class:`~discord.ext.commands.Flag`
+        The flag that received too many values.
+    values: List[:class:`str`]
+        The values that were passed.
+    """
+    def __init__(self, flag, values):
+        self.flag = flag
+        self.values = values
+        super().__init__(f'Too many flag values, expected {flag.max_args} but received {len(values)}.')
+
+class BadFlagArgument(FlagError):
+    """An exception raised when a flag failed to convert a value.
+
+    """
+    def __init__(self, flag):
+        self.flag = flag
+        try:
+            name = flag.annotation.__name__
+        except AttributeError:
+            name = flag.annotation.__class__.__name__
+
+        super().__init__(f'Could not convert to {name!r} for flag {flag.name!r}')
+
+class MissingRequiredFlag(FlagError):
+    """An exception raised when a required flag was not given.
+
+    This inherits from :exc:`FlagError`
+
+    .. versionadded:: 2.0
+
+    Attributes
+    -----------
+    flag: :class:`~discord.ext.commands.Flag`
+        The required flag that was not found.
+    """
+    def __init__(self, flag):
+        self.flag = flag
+        super().__init__(f'Flag {flag.name!r} is required and missing')
+
+class MissingFlagArgument(FlagError):
+    """An exception raised when a flag did not get a value.
+
+    This inherits from :exc:`FlagError`
+
+    .. versionadded:: 2.0
+
+    Attributes
+    -----------
+    flag: :class:`~discord.ext.commands.Flag`
+        The flag that did not get a value.
+    """
+    def __init__(self, flag):
+        self.flag = flag
+        super().__init__(f'Flag {flag.name!r} does not have an argument')