Problem 1: Entering a command of `:::save` gives an error.
Problem 2: Entering a command of `:save\n` gives an error.
Both scenarios may seem a bit silly at first, but I encountered both by
copy/pasting a command:
1. Enter `:` in qutebrowser.
2. Copy a full line from a terminal starting with `:`.
3. You will now have both of the above problems.
Solution: Trim all whitespace and `:` of a command. This is also what
Vim does, by the way.
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