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.

Tools/mavlink_shell.py

@@ -16,7 +16,7 @@ from argparse import ArgumentParser
 try:
     from pymavlink import mavutil
 except ImportError as e:
-    print("Failed to import pymavlink: " + e)
+    print("Failed to import pymavlink: " + str(e))
     print("")
     print("You may need to install it with:")
     print("    pip3 install --user pymavlink")
@@ -26,7 +26,7 @@ except ImportError as e:
 try:
     import serial
 except ImportError as e:
-    print("Failed to import pyserial: " + e)
+    print("Failed to import pyserial: " + str(e))
     print("")
     print("You may need to install it with:")
     print("    pip3 install --user pyserial")

Tools/mavlink_ulog_streaming.py

@@ -17,7 +17,7 @@ from argparse import ArgumentParser
 try:
     from pymavlink import mavutil
 except ImportError as e:
-    print("Failed to import pymavlink: " + e)
+    print("Failed to import pymavlink: " + str(e))
     print("")
     print("You may need to install it with:")
     print("    pip3 install --user pymavlink")

Tools/serial/generate_config.py

@@ -10,7 +10,7 @@ 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")
@@ -20,7 +20,7 @@ except ImportError as e:
 try:
     import yaml
 except ImportError as e:
-    print("Failed to import yaml: " + e)
+    print("Failed to import yaml: " + str(e))
     print("")
     print("You may need to install it using:")
     print("    pip3 install --user pyyaml")

Tools/upload_log.py

@@ -17,7 +17,7 @@ import sys
 try:
     import requests
 except ImportError as e:
-    print("Failed to import requests: " + e)
+    print("Failed to import requests: " + str(e))
     print("")
     print("You may need to install it using:")
     print("    pip3 install --user requests")

Tools/validate_yaml.py

@@ -4,13 +4,12 @@
 from __future__ import print_function
 
 import argparse
-import os
 import sys
 
 try:
     import yaml
 except ImportError as e:
-    print("Failed to import yaml: " + e)
+    print("Failed to import yaml: " + str(e))
     print("")
     print("You may need to install it using:")
     print("    pip3 install --user pyyaml")
@@ -20,7 +19,7 @@ except ImportError as e:
 try:
     import cerberus
 except ImportError as e:
-    print("Failed to import cerberus: " + e)
+    print("Failed to import cerberus: " + str(e))
     print("")
     print("You may need to install it using:")
     print("    pip3 install --user cerberus")