Reject a truncated exponent instead of keeping the coefficient

[SashaMIT]

Summary

• SetString("1e") returned 1. SetString("1e+") and SetString("1e-") also returned 1, and SetString(".") returned 0. None of them set ConversionSyntax.
• A truncated exponent, and a lone dot, are now quiet NaNs with ConversionSyntax, which is the same path other rejected numeric strings already take.
• 1e2, 1e+2, 1e-1, 1., and .5 still parse as before.

Test plan

• TestSetStringTruncatedNumeric failed on tip: 1e = 1, want NaN
• The same test passes after the change
• Parser, format, compare, ceil, and floor tests around this path pass

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