$ \int_{-\infty}^a \exp(-t^2)\, \text dt = \\ \frac{\sqrt{\pi}}{2}+\int_0^a\exp(-t^2)\, \text dt=\\ \frac{\sqrt{\pi}}{2}+\int_0^a\sum_{n=0}^\infty \frac{(-1)^n t^{2n}}{n!} \, \text dt =\\ \frac{\sqrt{\pi}}{2}+\sum_{n=0}^\infty \frac{(-1)^n a^{2n+1}}{n!(2n+1)} $
You can use this to estimate the integral to sufficient accuracy (as GEdgar mentioned, however, this sum converges well when $a \approx 0$ but much slower when $a$ is large). As was mentioned, no simple "closed form" for this integral exists.
For example, we may approximate $\int_{-\infty}^1 \exp(-t^2)\, \text dt$ with a truncated (6 terms) series:
$\int_{-\infty}^1 \exp(-t^2)\, \text dt =1.63305\cdots \approx \frac{\sqrt{\pi}}{2}+\sum_{n=0}^6 \frac{(-1)^n}{n!(2n+1)}=1.66306\cdots$
See also this paper for more strategies.