Tools: make Python import error more readable

The problem with printing the exception was that starting with
Python 3.6 the ImportError is yet another (sub) exception called
ModuleNotFoundError which can't be printed as a string and then triggers
another exception:

```
Traceback (most recent call last):
  File "/home/julianoes/src/Firmware/Tools/serial/generate_config.py", line 11, in <module>
    import jinja2
ModuleNotFoundError: No module named 'jinja2'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/julianoes/src/Firmware/Tools/serial/generate_config.py", line 13, in <module>
    print("Failed to import jinja2: " + e)
TypeError: must be str, not ModuleNotFoundError
```

As per @bkueng's suggestion the easiest is to cast the exception to str
and that way prevent the second exception.

src/lib/mixer/MultirotorMixer/geometries/tools/px_generate_mixers.py

@@ -37,28 +37,34 @@ px_generate_mixers.py
 Generates c/cpp header/source files for multirotor mixers
 from geometry descriptions files (.toml format)
 """
+import sys
 
 try:
     import toml
+except ImportError as e:
+    print("Failed to import toml: " + str(e))
+    print("")
+    print("You may need to install it using:")
+    print("    pip3 install --user toml")
+    print("")
+    sys.exit(1)
+
+try:
     import numpy as np
 except ImportError as e:
-    print("python import error: ", e)
-    print('''
-Required python3 packages not installed.
-
-On a GNU/Linux or MacOS system please run:
-  sudo pip3 install numpy toml
-
-On Windows please run:
-  easy_install numpy toml
-''')
-    exit(1)
+    print("Failed to import numpy: " + str(e))
+    print("")
+    print("You may need to install it using:")
+    print("    pip3 install --user numpy")
+    print("")
+    sys.exit(1)
 
 __author__ = "Julien Lecoeur"
 __copyright__ = "Copyright (C) 2013-2017 PX4 Development Team."
 __license__ = "BSD"
 __email__ = "julien.lecoeur@gmail.com"
 
+
 def parse_geometry_toml(filename):
     '''
     Parses toml geometry file and returns a dictionary with curated list of rotors

src/lib/parameters/px_generate_params.py

@@ -11,7 +11,7 @@ import sys
 try:
     from jinja2 import Environment, FileSystemLoader
 except ImportError as e:
-    print("Failed to import jinja2: " + e)
+    print("Failed to import jinja2: " + str(e))
     print("")
     print("You may need to install it using:")
     print("    pip3 install --user jinja2")