Bikeshedding: Error messages and comments

I'm sorry for the major bikeshedding, but let's settle this once and for all to avoid future bikeshedding in PRs.

How do we want error messages to look like?

1. Should the first word be capitalized or not?
2. Punctuation: When the message is just one sentence or a sentence fragment, should we end it with a period or exclamation mark, or no punctuation?

In Julia, sentence fragments starting with lowercase and no punctuation seem to be preferred:

julia> Dict{1,2 ,3 }
ERROR: too many parameters for type

julia> if 4
       end
ERROR: TypeError: non-boolean (Int64) used in boolean context

julia> sqrt(-1)
ERROR: DomainError with -1.0:
sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).
Stacktrace:
 [1-2] ⋮ internal
     @ Base.Math, Unknown
   [3] sqrt(x::Int64)
     @ Base.Math ./math.jl:1372
Use `err` to retrieve the full stack trace.

julia> f()
ERROR: UndefVarError: f not defined

julia> Int(1e100)
ERROR: InexactError: Int64(1.0e100)
Stacktrace:
   [1] Int64(x::Float64)
     @ Base ./float.jl:788

julia> x = []; x[1]
ERROR: BoundsError: attempt to access 0-element Vector{Any} at index [1]
Stacktrace:
   [1] getindex(A::Vector{Any}, i1::Int64)
     @ Base ./array.jl:924

But not consistent outside of base:

julia> using sfdljoisdf
ERROR: ArgumentError: Package sfdljoisdf not found in current path.
- Run `import Pkg; Pkg.add("sfdljoisdf")` to install the sfdljoisdf package.

Same for comments, while we're at it:

1. Should comments start with a lowercase letter?
2. Should sentence fragments end with a period?
3. Should full sentences end with a period? (Of course, when the comment consists of multiple sentences, we end all of them with a period.)

My personal opinion (when I say no opinion, I still want us to pick one and use it consistently):

• No opinion about starting error messages with lower or upper case.
• No opinion about ending without punctuation or with a period, but I am strongly against exclamation marks (too aggressive).
• I prefer starting comments with an uppercase letter.
• I prefer no punctuation for comments unless there are multiple sentences, in which case I end all of them with a period. (I prefer not to add periods after links to avoid breaking the link when copying it.)

No files, tests, or entry points are named. Start by reading the issue and its discussion, then survey the project's existing error messages and comments; done means the project has an agreed, documented convention that can be applied consistently.