Repository: PlasmaPy/PlasmaPy

Issue:
Coulomb_logarithm raises RelativityError instead of RelativityWarning
The [docs mention that](http://plasmapy.readthedocs.io/en/latest/api/plasmapy.physics.transport.Coulomb_logarithm.html#coulomb-logarithm) `physics.transport.Coulomb_logarithm` raises a `RelativityWarning` if the input velocity is greater than 5% of the speed of light.

But in actual it results in `RelativityError` instead of `RelativityWarning`:

```python
>>> from plasmapy.physics.transport.collisions import Coulomb_logarithm
>>> import astropy.units as u
>>> n = 1e19*u.m**-3
>>> T = 1e6*u.K
>>> V = 4e8*u.m/u.s
>>> particles = ('e', 'p')
>>> Coulomb_logarithm(T, n, particles, V=V)
```
```
RelativityError: V is yielding a velocity that is 1.334 times the speed of light.
```


Discussion:
Working as intended! Check relativistic is supposed to warn at speeds from 0.05 to 1 c and raise an error above that. Perhaps the error message could be reworded to say it's, for example, 7% c for a warning and 120%c for an error.
Ah, I see. Yep, I think it would be nice to add `RelativityError` along with `ValueError`, `UnitConversionError`, etc. in errors raised.
I remember wondering why we didn't have one! Wanna try? That should be the only place it's going to pop up right now.
This seems to be the case with most functions in [`collisions.py`](https://github.com/PlasmaPy/PlasmaPy/blob/166747196434232946086a36ae916fbfc65d6bc6/plasmapy/physics/transport/collisions.py). I'd be happy to make a PR!

Resolving patch:
diff --git a/plasmapy/physics/transport/collisions.py b/plasmapy/physics/transport/collisions.py
--- a/plasmapy/physics/transport/collisions.py
+++ b/plasmapy/physics/transport/collisions.py
@@ -84,6 +84,10 @@ def Coulomb_logarithm(T,
     PhysicsError
         If the result is smaller than 1.
 
+    RelativityError
+        If the input velocity is same or greater than the speed
+        of light.
+
     Warns
     -----
     ~astropy.units.UnitsWarning
@@ -311,6 +315,10 @@ def b_perp(T,
     TypeError
         If T, or V are not Quantities.
 
+    RelativityError
+        If the input velocity is same or greater than the speed
+        of light.
+
     Warns
     -----
     ~astropy.units.UnitsWarning
@@ -415,6 +423,10 @@ def impact_parameter(T,
     TypeError
         If the n_e, T, or V are not Quantities.
 
+    RelativityError
+        If the input velocity is same or greater than the speed
+        of light.
+
     Warns
     -----
     ~astropy.units.UnitsWarning
@@ -605,6 +617,10 @@ def collision_frequency(T,
     TypeError
         If the n_e, T, or V are not Quantities.
 
+    RelativityError
+        If the input velocity is same or greater than the speed
+        of light.
+
     Warns
     -----
     ~astropy.units.UnitsWarning
@@ -1012,6 +1028,10 @@ def mean_free_path(T,
     TypeError
         If the n_e, T, or V are not Quantities.
 
+    RelativityError
+        If the input velocity is same or greater than the speed
+        of light.
+
     Warns
     -----
     ~astropy.units.UnitsWarning
@@ -1087,7 +1107,6 @@ def Spitzer_resistivity(T,
         and electron-ion collisions, and the ion temperature for
         ion-ion collisions.
 
-
     n : ~astropy.units.Quantity
         The density in units convertible to per cubic meter.
         This should be the electron density for electron-electron collisions,
@@ -1130,6 +1149,10 @@ def Spitzer_resistivity(T,
     TypeError
         If the n_e, T, or V are not Quantities.
 
+    RelativityError
+        If the input velocity is same or greater than the speed
+        of light.
+
     Warns
     -----
     ~astropy.units.UnitsWarning
@@ -1254,6 +1277,10 @@ def mobility(T,
     TypeError
         If the n_e, T, or V are not Quantities.
 
+    RelativityError
+        If the input velocity is same or greater than the speed
+        of light.
+
     Warns
     -----
     ~astropy.units.UnitsWarning
@@ -1378,6 +1405,10 @@ def Knudsen_number(characteristic_length,
     TypeError
         If the n_e, T, or V are not Quantities.
 
+    RelativityError
+        If the input velocity is same or greater than the speed
+        of light.
+
     Warns
     -----
     ~astropy.units.UnitsWarning
@@ -1492,6 +1523,10 @@ def coupling_parameter(T,
     TypeError
         If the n_e, T, or V are not Quantities.
 
+    RelativityError
+        If the input velocity is same or greater than the speed
+        of light.
+
     Warns
     -----
     ~astropy.units.UnitsWarning


Test patch: