  A goal of forthcoming imaging surveys is to use weak gravitational lensing
shear measurements to constrain dark energy. We quantify the importance of
uncalibrated photometric redshift outliers to the dark energy goals of
forthcoming imaging surveys in a manner that does not assume any particular
photometric redshift technique or template. In so doing, we provide an
approximate blueprint for computing the influence of specific outlier
populations on dark energy constraints. We find that outliers whose photo-z
distributions are tightly localized about a significantly biased redshift must
be controlled to a per-galaxy rate of <~ a few times 10^-3 to insure that
systematic errors on dark energy parameters are rendered negligible. In the
complementary limit, a subset of imaged galaxies with uncalibrated photometric
redshifts distributed over a broad range must be limited to fewer than a
per-galaxy error rate of <~ a few times 10^-4. Additionally, we explore the
relative importance of calibrating the photo-z's of a core set of relatively
well-understood galaxies as compared to the need to identify potential
catastrophic photo-z outliers. We discuss the degradation of the statistical
constraints on dark energy parameters induced by excising source galaxies at
high- and low-photometric redshifts, concluding that removing galaxies with
z_phot >~ 2.4 and z_phot <~ 0.3 may mitigate damaging catastrophic redshift
outliers at a relatively small (~ 20%) cost in statistical error. In an
appendix, we show that forecasts for the degradation in dark energy parameter
constraints due to uncertain photometric redshifts depend sensitively on the
treatment of the nonlinear matter power spectrum. Previous work using PD96 may
have overestimated the photo-z calibration requirements of future surveys.
